One of the most famous scenes in 'Goodfellas' is based on something that actually happened to Joe Pesci
One of the most memorable scenes in "Goodfellas" occurs early on, when the audience is introduced to most of the crew at the Bamboo Lounge. Henry Hill (Ray Liotta) and Tommy DeVito (Pesci) get into a tense exchange in which Pesci asks "Funny how?"
Apparently, this scene was heavily improvised and based off of a story Pesci told at rehearsal.
A young Pesci was a waiter at a restaurant and told a mobster that he was funny. Needless to say, the wiseguy didn't take this compliment too well.
In the film's scene, DeVito seems to get offended after Hill calls him funny. "Am I a clown? Do I amuse you?" he asks.
Scorsese liked this story so much that he allowed Pesci to bring it into the movie. However, he did not tell anyone outside of Pesci and Liotta that the scene would be improvised, because he wanted to see their genuine surprised reactions.
According to a making-of documentary, Scorsese decided to shoot this scene with only medium takes and no closeups, because he wanted to see everybody's genuine reactions.
"You don't improvise on camera when we're shooting," Pesci said in an interview during a making of documentary, "They all think that Marty [Scorsese] doesn't do anything, that he lets the actors say 'Okay, go ahead' and enjoys it. It's not true. It's crazy to think you can go in there and make a movie like that. It has to be structured."
Pesci won the Academy Award for best supporting actor for his role in "Goodfellas."
You can watch the (very NSFW) memorable clip below: