Marlon Brando wanted to make Don Corleone 'look like a bulldog', so he stuffed his cheeks with cotton wool for the audition. For the actual filming, he wore a mouthpiece made by a dentist. This appliance is on display in the American Museum of the Moving Image in Queens, New York.
James Caan improvised the part where he throws the FBI photographer to the ground. The extra's frightened reaction is genuine. Caan also came up with the idea of throwing money at the man to make up for breaking his camera. As he put it, "Where I came from, you broke something, you replaced it or repaid the owner."
The scenes in which Enzo comes to visit Vito Corleone in the hospital were shot in reverse, with the outside scene shot first. Gabriele Torrei, the actor who plays Enzo, had never acted in front of a camera before and his nervous shaking, after the car drives away, was real.