During an early shot of the scene where Vito Corleone returns home and his people carry him up the stairs, Marlon Brando put weights under his body on the bed as a prank, to make it harder to lift him.
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. Director Francis Ford Coppola liked the genuine nervousness and used it in the final cut. The scenes of Luca practicing his speech were added later.
Cinematographer Gordon Willis earned himself the nickname "The Prince of Darkness," since his sets were so underlit. "Paramount Pictures" executives initially thought that the footage was too dark, until persuaded otherwise by Willis and Francis Ford Coppola that it was to emphasize the shadiness of the Corleone family's dealings.
Francis Ford Coppola held improvisational rehearsal sessions that simply consisted of the main cast sitting down in character for a family meal. The actors and actresses couldn't break character, which Coppola saw as a way for the cast to organically establish the family roles seen in the final film.
The cat held by Marlon Brando in the opening scene was a stray that Coppola found while on the lot at "Paramount Pictures," and was not originally called for in the script. So content was the cat that its purring muffled some of Brando's dialogue and, as a result, most of his lines had to be looped.
Gray Frederickson: The film's associate producer as the cowboy in the studio when Tom Hagen encounters Studio Head Woltz for the first time.