With a cinematic universe as big as the MCU, it would be easy to presume that all the movies are strictly written and there's no space for surprise. However, even in the MCU, the actors sometimes get the opportunity to offer their input and change the scene or scenes they're playing.
From small cosmetic changes to multiple improvised dialogues, the actors appearing in the MCU did it all. Some of them are then better skilled at improv than others. The proof is that if the fans didn't know which scene or dialogue was improvised, they never would have guessed it otherwise.
Curious Nick Fury
Nowadays, the superspy Nick Fury is having aliens over for breakfast. But when he first met Carol Danvers in Captain Marvel (2019), he had no idea that aliens were real. Fury learned it the hard way when one of the Skrulls impersonated Coulson and attacked him. Later on, Fury attends the Skrull's autopsy, and can't help his curiosity. He takes a peek at the Skrull's... intimate organs, let's say. Samuel L. Jackson improvised Fury's peeking under the sheet covering the body, and the creators liked it so they kept it in the movie.
Tony Stark Is Iron Man
Unlike the comics, where Iron Man hides his true identity and pretends to be Tony Stark's bodyguard, Stark doesn't keep his superhero activity a secret in the MCU. In fact, he confesses being Iron Man in the first film's finale. A lot of the dialogue in Iron Man (2008) was improvised but one of the most important moments was when Robert Downey Jr. said: "I am Iron Man." The line was Downey Jr.'s idea and it stayed with Tony Stark until Avengers: Endgame (2019) - since he said the exact same thing moments before he snapped his fingers and defeated Thanos once and for all.
Spider-Man Dies
Avengers: Infinity War (2018) killed a lot of characters - but Spider-Man's death was one of the most tragic moments that made multiple members of the audience cry. But what might people not know is that Tom Holland and Robert Downey Jr. improvised the scene and the dialogue in it. They only knew Spider-Man would die but the rest was up to them. Holland's now-iconic line: "I don't want to go!" is then the young actor's invention.
The Friendship Between Iron Man And Spider-Man
Peter Parker and Tony Stark built one of the most iconic friendships in the MCU - as Tony was a father-figure of sorts for Peter. Besides Spider-Man's death scene, there are also other, more relaxing improvised moments between them. One such moment happens when Peter goes for the hug in the car and Tony responds: "That's not a hug. I'm just grabbing the door for you." Downey Jr. came up with the line.
And he also urged Tom Holland to move aside when the young actor forgot to make space for Downey Jr. while they talk in Peter's bedroom. Downey Jr. goes to sit, Holland forgets to move aside, and Downey Jr. saves the day by saying: "I'm gonna sit here, so you move the leg."
The Mystery Of Gamora
Speaking of iconic lines, Drax has some of the funniest lines in the Guardians of the Galaxy movies. But his best line, by far, came in Avengers: Infinity War - and it wasn't even in the script! When Star-Lord asks "Where is Gamora?" and Iron Man follows up with a confused "Who is Gamora?", Drax's reaction takes both of them by surprise: "I'll do you one better. Why is Gamora?" Why indeed, that's the question.
The Unusual Friendship
Hawkeye and Quicksilver have a love-hate relationship in Avengers: Age of Ultron (2015). Quicksilver originally stands against the Avengers and he makes fun of Hawkeye. But in the end, he saves Hawkeye's life by sacrificing his own. Before that happened, however, Quicksilver drove Hawkeye crazy. And Jeremy Renner demonstrated it by considering getting rid of Quicksilver in the following improvised monologue: "Nobody would know. Nobody. Last time I saw him Ultron was sitting on him. Yeah, he'll be missed. That quick little bastard, I miss him already." If anybody could pull this lie off, it'd be Hawkeye.
Long Time No See
By the time Thor and Captain America reunite in Avengers: Infinity War, they haven't seen each other for a few years. They meet in Wakanda where Thor shows up out of nowhere and joins the battle against Thanos' forces.
The dialogue between Thor and Captain America was improvised but could very well happen between two friends who lost touch with each other for a while. Captain asks Thor if he got a new haircut and Thor then points out that Captain copied his beard.
Tony Stark Loves His Shawarma
Almost every movie in the MCU has one or more post-credit scenes. But rarely any of the show the Avengers just hanging out and eating. The famous shawarma scene was based on an improvised line by Robert Downey Jr. After Iron Man flies the nuke into space and falls unconscious, he mentions he would like to get some shawarma when he wakes up. That resulted in the post-credit scene which was shot later than the rest of the Avengers (2012).
Almost All Of Thor: Ragnarok
Thor: Ragnarok (2017) is one of the funniest movies in the MCU - possibly because almost all of it, about 80 percent of the film, was improvised by the actors. One such example is the "Get help" scene between Thor and Loki - a clever strategy for how to confuse and defeat their enemies. Loki isn't too happy about it but Thor gets his way in the end. And there's also Jeff Goldblum aka the villainous Grandmaster who improvised a majority of his dialogue.
+ Bonus: The Scenes That Didn't Make It Into The Films
Finally, there are two improvised scenes that didn't make it into the films - but both are hilarious and include kisses. In Avengers: Age of Ultron (2015), Chris Evans went for a kiss but considering that Tony Stark and Steve Rogers argue throughout most of the film, the scene ended up in bloopers. And in Avengers: Endgame, Stark kisses Peter Parker on the cheek once they reunite instead of just hugging him. It was a dad-like moment from Stark but once again, the creators went with a different take of the scene.