1.In Avengers: Infinity War, Tom Holland improvised his final lines before disintegrating and turning to dust.
2.In Willy Wonka & the Chocolate Factory, Gene Wilder improvised his character's limp + somersault during Wonka's opening entrance.
3.In The Hunger Games: Catching Fire, Elizabeth Banks purposefully went off-script to say that the Capitol's library was made of mahogany, which referenced her famous line from the first movie.
4.In Fast & Furious 6, The Rock ad-libbed this insult, making Ludacris literally spit out his drink from laughing too hard.
5.In Harry Potter and the Chamber of Secrets, Tom Felton, aka Draco Malfoy, forgot his line while shooting and improvised an even better one.
6.In Call Me By Your Name, Timothée Chalamet improvised that final bit where his character looked directly into the camera's lens.
7.In The Usual Suspects, the lineup scene was supposed to be done straight-faced, but Benicio Del Toro kept farting and no one could get through the scene without laughing.
8.In Iron Man, Robert Downey Jr. deviated from the script and ad-libbed the famous "I am Iron Man" line.
9.In Roman Holiday, legend had it that if a liar stuck their hand in the Mouth of Truth monument, it would be bitten off, so Gregory Peck stuck his hand inside, screamed, and scared the heck out of Audrey Hepburn.
10.In Goodfellas, Joe Pesci improvised the whole "funny how" scene with Ray Liotta based on a story he told during rehearsal.
11.In Midnight Cowboy, a cab driver almost ran over Dustin Hoffman during the shot, to which he slammed on the car and yelled, "I'm walkin' here!"
12.In The 40-Year-Old Virgin, Steve Carell's chest was waxed for the very first time, and he didn't think it was going to hurt, so all of his reactions were genuine.
13.In Spider-Man: Homecoming, Tom Holland and Robert Downey Jr. improvised that whole "hug" scene.
14.In Django Unchained, Leo DiCaprio accidentally cut his hand open while filming a scene but decided to keep acting anyway, even smearing the blood all over Kerry Washington's face.
15.In Clue, Madeline Kahn's "flames on the side of my face" speech was the only improvised line in the entire movie.
16.In Alien, only John Hurt knew that the alien was going to explode out of his chest, so the other actors' horrified reactions were absolutely real, even making one actress faint.
17.In To All The Boys I've Loved Before, Noah Centineo improvised that glorious pocket spin, which immediately made everyone swoon.
18.In You've Got Mail, Tom Hanks exited the bookstore while carrying balloons and a goldfish, but the balloons accidentally got caught in the door, so he said, "Good thing it wasn't the fish."
19.In Jaws, upon seeing the great white shark for the first time, Roy Scheider ad-libbed one of the most iconic lines in movie history.
20.In Blade Runner, Rutger Hauer improvised most of his "tears in rain" speech at the end of the movie, which actually made the screenwriter a little jealous.
21.In Clueless, Donald Faison improvised "I'm keepin' it real" during the head shaving scene, based on something he heard a kid in his neighborhood say.
22.In Mission: Impossible – Fallout, Henry Cavill improvised that double-loaded arm punch, and the director liked it so much he made him do it in every take.
23.And in Aladdin, Robin Williams went off-script allllll the time, and the animators actually loved it.
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