Lenny Montana (Luca Brasi) was so nervous about working with Marlon Brando that, in the first take of their scene together, he flubbed some lines. Francis Ford Coppola liked the genuine nervousness and used it in the final cut. The scenes of Brasi practicing his speech were added later.
Marlon Brando wanted to make Don Corleone "look "like a bulldog," so he stuffed his cheeks with cotton wool for the audition. For 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.
Although there are many claims of real Mafiosi as cast members Francis Ford Coppola stated in a May 2009 interview with Howard Stern that no organized crime members were cast or used as consultants. Coppola went on to explain there are expectations of reciprocity once one is provided a "favor" by an organized crime member or otherwise involved in a business action with the same. He specifically denied the connection of Gianni Russo to organized crime. The closest Coppola claims to have come to a real gangster during production, at least to his knowledge, was an interaction with Lenny Montana, who played Luca Brasi. Coppola said when he asked if Montana knew how to spin the cylinder of the revolver Montana replied "You kiddin'?"