According to Henry Hill, whose life was the basis for the book and film, Joe Pesci's portrayal of Tommy DeSimone was 90-99% accurate, with one notable exception; the real Tommy DeSimone was massively built.
Robert De Niro wanted to use real money for the scene where Jimmy hands out money, because he didn't like the way fake money felt in his hands. The prop master gave De Niro $5,000 of his own money. At the end of each take, no one was allowed to leave the set until all the money was returned and counted.
The "How am I funny?" scene is based on something that actually happened to Joe Pesci. While working in a restaurant, a young Pesci apparently told a mobster that he was funny, a compliment that was met with a less-than-enthusiastic response. Pesci relayed the anecdote to Martin Scorsese, who decided to include it in the film. Scorsese didn't include the scene in the shooting script, so that Pesci and Ray Liotta's interactions would elicit genuinely surprised reactions from the supporting cast.
According to Nicholas Pileggi, some mobsters were hired as extras to lend authenticity to scenes. The mobsters gave Warner Bros. fake Social Security numbers, and no one knows how they received their paychecks.
Martin Scorsese first got wind of Nicholas Pileggi's book "Wiseguy" when he was handed the galley proofs. Although Scorsese had sworn off making another gangster movie, he immediately cold-called the writer and told him; "I've been waiting for this book my entire life." Pileggi replied; "I've been waiting for this phone call my entire life."
Joseph Bono: Mikey Franzese, who appears briefly as the camera pans through the Bamboo Lounge near the start of the movie. Mikey's only line is, "I haven't saw that guy. Yeah, I wanna see him."