The biblical quote at the end of the film (“All I know is this; once I was blind, and now I can see”) was a reference to Martin Scorsese’s film professor, to whom the film was dedicated. The man died just before the film was released. Scorsese credits his professor with helping him “to see”.
Martin Scorsese was worried about the On the Waterfront recitation because he knew he’d be inviting critical comparison between the scene in this film and the original film’s scene. Robert De Niro read it in various ways. Scorsese chose the take in which the recitation is extremely flat specifically to mute the comparison, and to suggest that it is simply a recitation and not indicative of how Jake LaMotta felt about his brother.
When the real Jake LaMotta saw the movie, he said it made him realize for the first time what a terrible person he had been. He asked the real Vicki “Was I really like that?”. Vicki replied “You were worse.”
The later scenes with a more weightier La Motta were generally shot with the minimum of takes as Robert De Niro would become exhausted much more quickly.
Jake (Robert De Niro) asks Joey (Joe Pesci) “Did you fuck my wife?”. Director Martin Scorsese didn’t think that Pesci’s reaction was strong enough, so he asked De Niro to say “Did you fuck your mother?”
To visually achieve Jake’s growing desperation and diminishing stature, Martin Scorsese shot the later boxing scenes in a larger ring.
Sound effects for punches landing were made by squashing melons and tomatoes. Sound effects for camera flashes going off were sounds of gunshots. The original tapes were deliberately destroyed by the sound technicians, to prevent then being used again.
To show up better on black-and-white film, Hershey’s chocolate was used for blood.