Making movies is difficult, and if the crew wants to finish up on time, it's usually very important that the actors and filmmakers stick to the script. Sometimes, an opportunity for something even better comes up in the moment, and in the case of these 12 movies, they end up making the movie even better.
Raiders of the Lost Ark - Indy shoots a guy
Harrison Ford was supposed to film a fight scene but had caught dysentery while on set, meaning he was perpetually running to the bathroom. When shooting time came, he asked director Stephen Spielberg "Can I just shoot him?"
Dumb and Dumber - The most annoying sound in the world
A chunk of the scene was ad-libbed, but this part especially was all Jim Carrey
Zoolander - "But why male models?" (The second time)
Ben Stiller forgot his lines and accidentally repeated the one he'd just said, but David Duchovny ran with it, and it was so funny it made it into the movie.
Midnight Cowboy - "I'm walkin' here!"
A car that had been held up traffic due to the shooting of the movie tried to speed through, while Dustin Hoffman stayed in character.
The Empire Strikes Back - "I know"
The script called for Ford to say "I love you too" but he felt this suited Han Solo's rogueish character better. He was right.
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Jaws - "You're gonna need a bigger boat"
Roy Scheider improvised the line.
The Warriors - "Warriors! Come out to playyyyyyyyyyy!"
The line and the clanking of the beer bottles was entirely made up by actor David Patrick Kelly, and became the most iconic moment from the movie.
Goodfellas - The dinner scene
The entire scene in Tommy's mother's house was improvised, with Scorsese instructing all the actors (including his mother, who played Tommy's mother) to do what they wanted.
Aliens - "Game over, man! Game over!"
Bill Paxton was just supposed to say "What're we supposed to do?!" but added the other bit himself.
The 40 Year Old Virgin - The chest waxing scene
All of Steve Carrel's reactions to having his chest hair waxed are completely real.
The Godfather - "Leave the gun. Take the cannoli."
Only "leave the gun" was in the script, but actor Richard Castellano felt the scene needed something a bit extra.
The Shining - "Heeeeeeeeeeere's Johnny!"
The scene just called for Jack Nicholson to break the door down, but he decided to give it a bit of extra creepiness.