While Paramount brass dithered over whether to cast him as Michael Corleone, the role that would make him a star, a frustrated Al Pacino signed up for the role of Mario Trantino in this movie. When Paramount finally decided to offer him The Godfather (1972) role, they had to buy him out of his contract with Metro-Goldwyn-Mayer. Ironically, the role went to Robert De Niro, whom The Godfather Part II (1974) would also help to make a star.
Mario Trantino (Robert De Niro) was a thief. Ever the method actor, De Niro allegedly stole two shirts from Macy's department store and hid them under his jacket. De Niro and Leigh Taylor-Young were then apparently arrested. Reportedly, Producers Robert Chartoff and Irwin Winkler intervened and explained that it was not a real theft, and smoothed things over with the police.
The character of Kid Sally was based on real mobster Crazy Joe Gallo. Jerry Orbach was with Crazy Joe on his birthday but they separated prior to Gallo's arrival at Umberto's Clam House, where he was assassinated.
To prepare for his role as a newly arrived Italian immigrant, Robert De Niro went to Italy for a week to record voices of the accent he required.