Goodfellas had alot of improvised scenes the one I'm talking about in particular is the funny guy scene, In which all the dialogue in the scene was improvised by Joe Pesci, the reactions by Ray Liotta were genuine. Another note-worthy thing about this scene is that it was inspired by a similar real life incident in which Joe Pesci once called mobster funny. He told Martin Scorsese about this story, so he decided to include it in the movie, asking Ray Liotta to call him funny and the rest was come up with on the spot by Joe Pesci.