Goodfellas
The entire rant about being funny was made up on the day of shooting. The. Entire. Thing. From "I'm funny how? I mean, funny like I'm a clown? I amuse you? I make you laugh?" all the way down to Tommy declaring to Sonny "Yeah, alright. For $7,000, I charge you." The script just said to go on a rant, and Joe Pesci thankfully more than delivered on that front...