Nearly all of Vito's dialogue is in Sicilian. Robert De Niro spent four months learning to speak Sicilian.
Originally, the actors in the flashback scenes wore pants with zippers. One of the musicians pointed out that the zipper had not been invented at that time, so some scenes had to be re-shot with button-fly trousers.
Francis Ford Coppola had a horrible time directing The Godfather (1972), and asked to pick a different director for the sequel, while taking the title of producer for himself. He chose Martin Scorsese, whom the executives rejected. Thus, Coppola agreed to direct the film, with a few conditions.
The door to Vito Corleone's olive-oil business was rigged so that it would not open if a nail was inserted into the lock. Coppola kept this a secret from Leopoldo Trieste, who played Signor Roberto, and his difficulty in opening the door was real. Coppola wanted to film Trieste, a known Italian comedian, improvising his way through the scene. When Genco Abbandando opens the door, Frank Sivero surreptitiously pulls the nail out.
Hyman Roth's character is loosely based on real-life mobster Meyer Lansky. Lansky, who at the time of the film's release was living in Miami, reportedly phoned Lee Strasberg and said, "Now, why couldn't you have made me more sympathetic? After all, I am a grandfather."