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The scene where Robert De Niro's character shoots Don Fanucci strongly resembles the scene at the end of Taxi Driver (1976). In Taxi Driver (1976), he shoots a police officer in the face, and later in that scene, shoots the pimp (who's badly wounded but still alive, sitting on the floor with his back to the wall) in the mouth. In The Godfather Part II (1974), he shoots Don Fanucci in the face (at the exact spot he shot the police officer), and when he's on the floor with his back to the wall, very much in the same position as the pimp, De Niro shoots him again in the mouth.
Robert De Niro spent four months learning to speak the Sicilian dialect of Italian in order to play Vito Corleone. Nearly all of the dialogue that his character speaks in the film was in 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.
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."
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, who the film 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.