With over 100 villains in the Marvel universe, the superheroes have a lot on their plate. Saving the world isn't an easy task, but heroes like Iron Man, Thor, and the X-Men make it look like a breeze. When the entire universe is in trouble, however, the job gets a little bit tougher. Not all Marvel supervillains can pose a threat to entire galaxies, but there are a few that can. Of them, Thanos is undoubtedly one. Some protagonists aren't fit to take on the Mad Titan, but there is a handful that we think could finish him...
Member of the otherworldly and virtually immortal Asgardian race, mastery over the elements of thunder and lightning, and resistance to injury are only a few perks that come with Thor. If you watched Avengers: Infinity War, you witnessed how close Thor was to defeating Thanos once and for all.
In the comic books, the mighty warrior uses his hammer to beat Thanos to death along with the help of the Avengers. If there is anyone who can topple the purple giant, it's this guy right here.
Sometimes you have to get a witch to finish the job. Scarlet Witch is one of the more powerful Marvel characters. She carries a wide range of powers such as mind control, healing factor, control of elements, chaos magic, and much more. In the films, she is as strong as almost any other character.
She can also do something that many of the other heroes can accomplish-- destroying infinity stones. After figuring out how to crush those stones, she has what it takes to take down a gemless Thanos.
Jean Grey (Phoenix)
Out of all the X-Men members, Jean Grey with the Phoenix Force is possibly the most powerful character there is. We've seen her portrayed briefly in X-Men: Apocalypse and more so in X-Men: The Last Stand, but those versions of her showcase nowhere near her full potential.
In the Phoenix form, she can control time and manipulate matter. In pure form, she is also able to resurrect herself and others. Thanos might tremble in her presence even with all of the infinity stones.
After watching The Hulk get beat mercilessly by Thanos in Infinity War, some might find this entry surprising. We don't blame you because that fight was one-sided and it looked like the green beast couldn't hold a candle to the Mad Titan.
Due to The Hulk's inability to maintain focus, he can be his own worst enemy. But when he shows he is capable of controlling himself fully, few can stop him. In the comics, he's defeated Thanos by smashing him to a pulp.
Sorcery proves to be one of Thanos' weaknesses, and Doctor Strange has plenty of it! Strange's mastery of the mystic arts ranks him pretty high in the Marvel Universe as far as the most powerful go. With the correct strategy in place, Stange can topple his opponent.
We saw in Infinity War that Thor and Strange came the closest to defeating Thanos and had it not been for the infinity stones, the latter would have been successful.
Out of all the characters that you might see on this list, Iron Man is someone you might not have expected. Tony Stark might not have any powers as a human, but once that suit gets activated, his possibilities are endless.
In the film, we witnessed Iron Man get handled by Thanos, but it wasn't as easy as the Mad Titan thought it would be. Due to Stark's suit being able to transform into virtually any type of weapon, he has the power to come up with a sound plan being the genius he is, and effectively defeat Thanos.
Deadpool is such an intriguing character. He likes doing things just for fun, and he has a slew of powers that would prove extremely helpful in a battle against Thanos. One of his abilities is that he is effectively immortal. That sounds like something that could help against the villain.
One of his most head-scratching but useful powers is how he can break the 4th wall. That means he knows he's a comic book character and talks to the audience and can read old copies to find out someone's weakness. In the comics, he defeats all of the Marvel Universe just for giggles.
Drax The Destroyer
Have you ever wondered where Drax got his name? Drax the Destroyer was made with the sole purpose of defeating Thanos (hence the name "Drax the Destroyer"). Drax has a lot of powers that enable him to take down the menace.
The powers of Drax range from superior power to telepathic abilities. He is also capable of destroying a planet with only his strength. In 2006, he took down Thanos in the comics by ripping his heart out his chest. We haven't yet witnessed that type of power from Drax in the films, but we might see it soon.
Of course, the father of Thor would so graciously make this list. He's without a doubt one of the strongest in the Marvel Universe. Known as the god of wisdom and intelligence, those features give Odin an edge of The Mad Titan.
One of his powers includes transporting things into other dimensions as he showcased by doing it to an entire alien race. He could easily teleport any reinforcements Thanos might have away from the battle and use his time traveling technique to handle the villain.
It seems like whenever two powerful mutants have a child, their seed becomes stronger than both parents combined. That's the case for the son of Professor X, David Haller aka Legion. He's categorized as an Omega Level Mutant and for good reason.
One of his most surprising abilities is his split personalities. Each personality comes with a different set of powers and can take control of Haller's body completely. Of course, he can easily use his ability to warp reality at will.
It's already on record that Captain Marvel is the strongest Avenger in the Marvel Cinematic Universe. If the heroes hold any chance of defeating Thanos, she is a crucial component for sure. Her abilities are also off the charts.
Captain Marvel can fly six times the speed of light, she can sense things about to happen, and she has superhuman strength. She's defeated Thanos in the comics with the help of Drax. Captain Marvel doesn't sound like a person you want to fight in a battle.
Franklin Richards is the mutant son of Invisible Woman and Mr. Fantastic. Thanks to his super parents, his powers are sky high even as a child. He's capable of creating a pocket-sized universe whenever he wants. Some consider him the strongest mutant.
He is also immortal if that helps. Thanos has to use the infinity stones to make half of all life vanish. Richards can make Thanos disappear with just a thought. This matchup wouldn't be fair for Thanos.
Good luck with this one, Thanos. Owen Reese aka Molecule Man is one of the deadliest characters in Marvel. Reese became Molecule Man after he had an accident with Cosmic Cube energy. After that, he was able to control matter on a multiversal scale.
Molecule Man can easily defeat Thanos with one technique. All he has to do is rearrange the Mad Titan's body to whatever form he so wishes. That's a tough opponent for anyone.
As wild as this might appear, it isn't a joke. Yes, Marvel Comics has a superhero named Squirrel Girl who can communicate with the precious rodents. It would appear that her most formidable ability is superhuman agility, senses, and strength.
Add all that together and you get someone who defeated Thanos all alone in the comics. One can only imagine how other heroes who have tried to topple him felt after they discovered a girl who controls squirrels did it on her own.
Teased in Guardians of Galaxy 2, Adam Warlock is a superior superhero. His powers include superhuman strength, agility, and stamina, and he can perform energy and matter manipulation. Also, much like Doctor Strange, Adam Warlock has mystic powers.
Adam Warlock is a tough cookie to handle for any villain, but in the comics, Warlock teamed up with Drax the Destroyer and Silver Surfer to take down Thanos. He ends up taking his gauntlet in the process, allowing him to possess all the infinity stones.
Tony Stark's father Howard Stark created Arsenal during the days of World War II. Creating special robots runs in the family, but Arsenal is a very unique type of machine.
Arsenal can manipulate energies and send them to other dimensions. His sole purpose is to protect Tony at all costs, and in the cartoon TV show, he does that by beating Thanos with is special powers. The power of each infinity stone was useless against Arsenal.
Meet the Marvel copyright-friendly version of Superman. The only difference is that he sports a nice mohawk. Gladiator usually teams up with the X-Men, but since he is an alien, he spends a lot of his time in other galaxies.
Thanos once came to Gladiator's home planet to defeat his father. They threw everything at him, but it barely slowed Thanos down. Then, when Gladiator showed up, it only took one punch to knock out the purple beast.
The Pet Avengers
Yes, there is a Squirrel Girl and there are the Pet Avengers. There aren't many who can beat Thanos, but there are some who can make the Mad Titan seem like nothing more than a novice henchman.
The Pet Avengers were in charge of the infinity stones when Thanos came to Earth looking for them, so they started a fight. After getting through some cats and dogs, the Inhuman mutt Lockjaw used his powers to teleport the villain to another dimension.
We'll preface this one with saying it was more a team effort, but it wouldn't have happened without Wolverine. The clawed mutant tricked Thanos into becoming his advisor when he, The Hulk, Spider-Man, and Ghostrider became "The New Fantastic Four."
Upon gaining Thanos' trust, Wolverine cut off his arm that carried the Infinity Gauntlet. Thus, leaving the door wide open for all of the new Fantastic Four members to have it. The Hulk had a great time smashing Thanos as much as he could.
The award for worst father ever possibly goes to Thanos. Whether they are his adopted or biological children, they always want to finish him off and that includes his son, Thane. Born on Earth, Thanos invaded the planet at one time to find his hidden son.
With some help from Ebony Maw, Thanos' trusted assistant, Thane used his powers to encase his dad in a limbo-like cube of amber. Imagine going to look for your son only to find him and he effectively defeats you.
The Black Widow, also known as Natasha Romanova, was introduced in Tales of Suspense in 1964. She is a Russian spy and the antagonist of the superhero Iron Man. She later joins up with the United States and is a member of the spy agency S.H.I.E.L.D., and is also a member of the superhero team the Avengers.
In film, she has been featured in Iron Man 2, The Avengers, The Winter Soldier, Captain America: Civil War, among others.
Clinton Francis, or Hawkeye's, first appeared in Marvel comics as a villain in Tales of Suspense. However, shortly after, he joins up with the Avengers and has been a member ever since. At six years old, he ran away to join the circus with his brother where he becomes the assistant to the Swordsman.
As an apprentice, he grows to become a master archer and is referred to as "The World's Greatest Marksman" by the circus. After seeing Iron Man, he is inspired to become a costumed superhero himself.
Otherwise known as Peter Parker, Spider-Man first appeared in the comic book Amazing Fantasy in 1962. He is an orphan who was raised by his Aunt May and Uncle Ben in New York City after his parents tragically die in an airplane accident.
After he's bitten by a radioactive spider, he is able to climb walls, has heightened senses, and can even shoot webbing from his hand. Spider-Man has grown to become one of the most popular superheros of all time, being featured in countless TV shows and movies.
Black Panther's real name is T'Challa, the king and protector of the fantasy African nation called Wakanda. The Black Panther has heightened senses and improved strength which is achieved through Wakandan rituals such as drinking the heart-shaped herb.
Black Panther is the first superhero of African descent and has been featured in Marvel Comics since the mid-1960s. Recently, Black Panther has been in the movies Captain America: Civil War, Black Panther, Avengers: Infinity War, and Avengers: Endgame.
Luke Cage was created by Archie Goodwin, George Tuska, Ray Thomas, and John Romita Sr. After his creation in 1972, he was named the first black superhero to be the protagonist and title character of a comic book.
Cage is an ex-convict who was imprisoned for a crime that he did not commit, who has superhuman strength and unbreakable skin after voluntarily being subjected to an experimental procedure. He then becomes a "hero for hire" and teams up with the superhero Iron First.
Over time, a number of different characters have assumed the title of Ant-Man, many of whom have been connected with the Avengers. The first person to assume the title was Hank Pym, a Biophysicist and Security Operations Center expert. After his wife is murdered, he discovers a chemical substance that allowed him to alter his size and a helmet that he uses to control ants.
He later teamed up with Janet van Dyne who would become his girlfriend and the superhero the Wasp. The two would later go on to be founding members of the Avengers.
Jessica Jones is a superhero created by Brian Michael Bendis and Michael Gaydos. She first appeared in 2001, in which Jones is the owner and sole employee of Alias Private Investigation.
She attended high school with Peter Parker (Spider-Man) and is exposed to radiation while in a coma, which gave her super strength, invulnerability, and the power of flight. After watching Spider-Man fight against Sandman at school, she decided to use her powers for good and became a superhero herself.
Also known as Lieutenant Colonel James Rupert "Rhodey" Rhodes, War Machine first appeared in the Iron Man comic in 1979, with his battle armor giving him his name. He is a sidekick to Iron Man, who gives him his first suit mechanical suit.
He has been featured in numerous animated series and is portrayed by Don Cheadle in Iron Man 2, Iron Man 3, Avengers: Age of Ultron, Captain America: Civil War, Avengers: Infinity WatCaptain Marvel, among others.
Created by Steve Englehart and Steve Gan, Star-Lord, otherwise known as Peter Jason Quill, is an interplanetary policeman. His father, J'son crash landed on earth and was taken in by Meredith Quill and the two began a relationship with one another.
After leaving to go fight in a war on his home planet, he leaves Earth, unaware that Meredith is pregnant with his child. After Peter is born, Meredith is killed, leading Peter to grow up in an orphanage and later joins NASA where he becomes an interplanetary policeman.
Valkyrie is a superhero based on the Norse mythological figure Brynhildr. She is part of the Defenders and was a love interest of the superhero Thor. She is known by her Asgardian name Brunnhilde and was hand selected by Odin to be a shield maiden in his army.
She typically rides into battle on her winged horse name Aragorn and fights with an enchanted blade named Dragonfang. She is a founding member of the Secret Avengers and is the co-leader of the Fearless Defenders.
Vision is an android that was created by the robot Ultron to use against Ultron's own creator, Dr. Hank Pym. After learning that Ultron created him, Vision turned the Avengers against Ultron and became a member of the group as well. After joining the Avengers, Vision fights for control of his own mind to break free from Ultron.
Vision is portrayed by Paul Bettany in the films Avengers: Age of Ultron, Captain America: Civil War, and Avengers: Infinity War. He will also appear again for Marvel's 2019 movie, Avengers: Endgame.
Created by Bill Manto and Keith Giffen, Rocket Raccoon was introduced in 1976. He's a highly-intelligent, anthropomorphic raccoon who is also a very skilled marksman and military tactician. The character had a key role in the 2014 film Guardians of the Galaxy as well as other adaptations.
Rocket Raccoon is also featured in Guardians of the Galaxy Vol. 2, Avengers: Infinity War, and the upcoming Avengers: Endgame. The character is voiced by Bradley Cooper with motion capture provided by Sean Gunn.
Yon-Rogg is a commander and leader of Starforce, who is loyal to the Supreme Intelligence and the Kree Empire. In 1989, Rogg and Minn-Evra were dispatched to kill Mar-Vell on Earth. While on Earth, he met Vers, a United States Air Force Pilot, who he took under his wing and trained to become a weapon.
Yon-Rogg is the primary enemy of Captain Marvel and is indirectly responsible for turning Carol Danvers into Ms. Marvel. He is portrayed by Jude Law in the film Captain Marvel.
The Winter Soldier
During World War II, James Buchanan "Bucky" Barnes enlisted to fight for the United States in World War II. He trained as Captain America's partner while in the military, and the two fought against the Nazis as a duo known as the Invaders.
In the final days of the war, Barnes has an extremely close brush with death. The evil Arnim Zola recovered his body and erased his memory, turning him into the formidable Winter Soldier.
Wong is the sidekick of Sr. Strange and first appeared in the comic Strange Tales, yet remained unnamed for some time. He is a master of the martial and mystic arts, and in the miniseries Dr. Strange: The Oath, it is revealed that Wong comes from a long line of monks living in Kamar-Taj.
He also has a cousin that is training in the occult and has approached Wong saying that one day he might replace Wong as Dr. Strange's sidekick.
Loki is the son of King Laufey, the ruler of the Frost Giants in Jotunheim. After his birth, he was left to die, but was saved King Odin and his wife Frigga, who raised him as an Asgardian prince alongside with their other biological son, Thor.
Loki grew up to become the "God of mischief," and sabotaged Thor's coronation, which eventually led to Thor being exiled to Earth. However, Thor finally returns to end Loki's reign and rake the crown for himself.
Ebony Maw is a member of the Black Order, who is loyal to Thanos and helped him acquire the Infinity Stones. He is gifted with strong telekinetic powers and gives warnings from across the galaxy about Thanos growing power and their search for the stones.
However, after he had captured and was torturing Dr. Strange, he encountered Iron Man, who ended up killing Maw by ejecting him out of his own ship and into outer space.
Also known as Taneleer Tivan, the Collector is caretaker of the largest collection of interstellar fauna, relics, and other devices in the galaxy. He resides and operates oit of the Knowhere port installation.
Over the course of his life, his many dealings with Asgardians and the Guardians of the Galaxy led him to be in the possession of one of the Infinity Stones although he almost managed to acquire two. This led him to be attacked by Thanos who was searching from the stones at the tine.
Ronan The Accuser
Ronan the Accuser is the Supreme Accuser of the Kree Empire, the militaristic government of the Kree people. He is usually described as an enemy to many of the superhero teams including the Fantastic Four, the Avengers, and the Guardians of the Galaxy.
Ronan has been adapted from the comics through numerous platforms including animated television series and video games. Actor Lee Pace portrayed Ronan in the live action films in the Marvel Universe such as Guardians of the Galaxy and Captain Marvel.
Okoye is the general of the Dora Milaje, the primary force of Wakanda's army. After Erik Kilmonger took over Wakanda after overthrowing T'Challa, she was conflicted between remaining loyal to her friend or staying to serve her new king.
In the end, she makes the decision to rebel against Kilmonger and take back the throne. She was involved in the battle against Thanos' army, yet was unable to stop him from obtaining all of the Infinity Stones.
For a couple of years leading to the release of Avengers: Endgame, some fans tossed around a pretty weird theory about how Thanos could be killed. They suggested that Ant-Man could defeat Thanos easily, using his special powers that give him the ability to shrink and grow.
The theory holds that Ant-Man would first shrink down to ant size. Then, he would simply crawl into one of Thanos' ears (or another bodily orifice) and then expand inside the supervillain, killing him.
Dave Bautista Made Up The "Why Is Gamora" Line
Drax the Destroyer is meant to be a strong but relatively dim-witted character who takes himself way too seriously, AKA the perfect role for wrestler Dave Bautista. One of Drax's best lines from the MCU came from Avengers: Infinity War when he adds the line "I'll do you one better! Why is Gamora?"
The script was supposed to end with Robert Downey Jr.'s line "Who is Gamora?" but Bautista embodied Drax and added it in. After the improvisation, the film's screenwriter told Bautista "Okay, you're very good at your job."
"Hey Auntie" Was All Michael B. Jordan's Sass
Fans of Black Panther will never forget when Killmonger arrived in Wakanda and dropped some truth that he is the son of King T'Chaka's brother, N'Jobu. Since he's a descendant of N'Jobu, he technically had a claim to the throne to challenge T'Challa.
The scene was emotionally intense, but Michael B. Jordan imparted some of Killmonger's sass and improvised the line "Hey, Auntie." Unfortunately for Killmonger, it didn't exactly lighten the mood in the room.
Iron Man's Most Famous Line Was Completely Unscripted
One of the earliest unscripted lines in the MCU came in the very first Iron Man film. The now-iconic line "I am Iron Man" by Tony Stark was completely ad-libbed by RDJ. The line is now one of the most famous, and it did a lot to inspire the unscripted direction of Marvel movies.
Producer Kevin Feige said that RDJ's early ad-lib "inspired us to go further in trusting ourselves... and not being afraid to adapt and evolve" from the comic books.
Chris Hemsworth Decided To Try Hanging Mjölnir On The Coat Hook
So, apparently Chris Hemsworth is actually hilarious in real life, and the Marvel screenwriters did their best to let his humor shine through as Thor. They would often leave the camera rolling for longer than the scene and left many lines up to him.
One of the best scenes to come out of this strategy was from Thor: The Dark World when he casually hangs up his hammer Mjölnir on the coat rack. The move would have only been done by a God who doesn't think about how heavy the hammer truly is.
Benedict Cumberbatch Threw That Beyoncé Line At Benedict Wong
In Doctor Strange, the doctor pokes fun at the character Wong's name by calling him Beyoncé. Wong simply doesn't reply to the line and Dr. Strange goes on by asking Wong if he'd ever heard of the singer because she's a huge star on earth. He simply replies "No."
Wong said the line was totally improvised because Benedict Cumberbatch thought the name Wong seemed like Adele or Beyoncé. The director loved the improvisation and had Wong listening to Beyoncé in the next scene as "research."
Holland Ad-Libbed His Most Memorable (And Meme-able) Line
The internet was abuzz after Avengers: Infinity War was released and the iconic line "Mr. Stark? I don't feel so good..." gained full-blown meme status. Tom Holland knew he was one of the characters to get dusted, but his only direction was to "act like you don't want to go."
Holland's line improvisation worked perfectly since Tony Stark was a mentor and father figure to Peter Parker. Joe Russo loved what Holland did and called it "movie magic."
Jeremy Renner's "Nobody Would Know" Line Came After The Scene Cut
One of Hawkeye's most memorable lines came from Avengers: Age of Ultron. In one scene, he jokes about shooting Quicksilver in the back after he calls Hawkeye an old man. The scene was supposed to cut after the "old man" line, but Renner stopped running and pretending to shoot Quicksilver in the back while muttering the line "nobody would know."
The producers kept it in and now we all know what Hawkeye would have done if he could.
Thor's Love Of Coffee Is All Thanks To Hemsworth
Another classic Thor moment that is all thanks to Chris Hemsworth comes from the very first Thor film. In the coffee shop Thor is taken to, they order him a coffee, and after chugging the drink he ad-libbed the line, "This drink, I like it! Another!" and threw the cup on the ground.
It was actually the president of Marvel Studios, Kevin Feige, who loved the unscripted moment and requested the directors keep it in the film.
A Make-A-Wish Kid Inspired One Line
In Thor: Ragnarok, Thor gets excited once he sees The Hulk and tells everyone that The Hulk is a "friend from work." The line wasn't exactly ad-libbed because Hemsworth had it suggested to him by a kid who was visiting the set as part of Make-A-Wish.
When recalling that day of filming, Hemsworth said the Make-A-Wish kid was watching the scene play out and suggest Thor say it, and the directors instantly loved it. That's quite the wish come true!
Chris Pratt Got To Ad-Lib His Introduction
Guardians of the Galaxy director James Gunn already knew that actor Chris Pratt had some comedic chops, so he purposely didn't write much for Peter Quill's opening lines.
Pratt knew that Star-Lord was a relatively unknown character in the MCU, so he added his own disappointment when his captors didn't know who the legendary hero Star-Lord actually was. Pratt added his own humor to the rest of the film which made GOTG one of the funniest MCU films.
Hayley Atwell Grabbed Cap's Pecs Instinctively
In Captain America: The First Avenger, we all got to collectively swoon during the scene where Chris Evans takes off his shirt. In the movie, Hayley Atwell's character reaches up to feel Evans' perfectly sculpted pectorals. Atwell later said that move was complete instinct and was just her natural response to grab his pecs.
The director loved the natural act so much that they ended up doing a couple more takes with the move in it.
The Shawarma Scene Was An Afterthought
It might have been one of the more boring post-credit scenes, but the simplicity of the shawarma scene left a lasting impression in MCU history. The scene is all thanks to RDJ's improvised line at the end of The Avengers when he almost dies and suggests taking the day off and trying the shawarma place down the street.
The post-credit scene was filmed only a few days before the film's release because producers changed direction and wanted to highlight RDJ's funny line.
Winston Duke Improvised Those Gorilla Sounds
M'Baku is a pretty powerful guy just on his own, but in one scene of Black Panther, he adds some gorilla grunts to drive the point home. The actor, Winston Duke, improvised the sounds when Martin Freeman's character tries to step in and tell him off.
In the DVD commentary for Black Panther, the producer said the gorilla sounds worked perfectly because it was "M'Baku's way to say "nah, I'm not talking to you right now."
Thor And Loki's Childhood Story Was Ad-Libbed Multiple Times
Since the producers love Chris Hemsworth's humor so much, they purposely asked him to think up stories about what Thor and Loki's childhood was like. The director of the film revealed they did six different shots of that scene and Hemsworth came up with six different, equally hilarious stories.
The story that ended up in the film is about how Loki turned into a snake to stun Thor when they were only eight years old.
Tom Holland Hugged RDJ Just To See If He Could
In Spider-Man: Homecoming, Peter Parker and Tony Stark embrace in a weird hug in the car that we all loved, but Stark hated. According to Holland, that move was totally improvised because he just thought it would be funny to try and do it.
Holland said that RDJ "instincts are so good" then without skipping a beat, he brushed the hug off and said he was just getting the car door. Is there anything RDJ can't improvise as Tony Stark?
Thor And Captain America's Reunion Was All Chris Hemsworth
Chris Hemsworth strikes again with the hilarious improvisation. During the big battle scene from Avengers: Infinity War, Thor and Captain America reunite after years in the middle of the fight. Rather than exchange normal pleasantries, Hemsworth called out Chris Evans for copying his beard.
Considering that Evans' beard was actually a big deal for fans of the movies since it was Cap's first time with facial hair, the line ended up working perfectly. Thank you again, Chris Hemsworth, for being the comedic relief we all needed.
Tony Stark Is Always Eating Snacks Because RDJ Hides Them On Set
Fans of the MCU have noticed over the 22 films that Tony Stark loves to snack. Often in S.H.I.E.L.D. or Avengers meetings, Stark is seen snacking while half-listening to the others. That improvisation is thanks to the fact RDJ hides snacks all over the set. In fact, it's well known he hides snacks on every set he's on.
There are a few instances where mid-scene, he'll offer another cast member a snack. Luckily, his co-stars learned this habit and had no problem improvising during the scenes too.
RDJ Improvised A Line To Help Tom Holland With His Blocking
The DVD commentary on Spider-Man: Homecoming revealed that in the scene where Tony Stark is in Peter Parker's bedroom, Tom Holland completely blanked on his blocking. Rather than cut the scene to remind him where to move, RDJ simply ad-libbed the line "I'm gonna sit here, so you move the leg."
In the commentary, the producers reiterated just how talented RDJ is because not only did he use the line to help out the young Holland, but he made it "entertaining and fun."
The "Get Help" Scene Was All Chris Hemsworth's Idea
Hemsworth is obviously good an improvising lines, but he's also good at creating entire additional scenes. In Thor: Ragnarok, Thor pretends that Loki is hurt to ask for help from the guards in order to cause a distraction. Distracting the guards was written by the director, but throwing Loki's body at the guards was entirely thought up by Hemsworth.
That's one scene that needed a few takes though because it's a little difficult to improvise throwing a human body across the room.
Robert Downey Jr. Wasn't Supposed To Speak While Searching For The Secret Door
By the time Avengers: Age of Ultron was filmed, RDJ had cemented himself as being practically the same person as Tony Stark. During that film, all the script said was for Stark to be searching for a secret door, and they left the rest up to RDJ.
Rather than search silently, he added in the lines "Please be a secret door, please be a secret door... Yay!" While the muttering was funny, that "yay!" was an iconic Stark moment.