According to Mario Puzo, the character of Johnny Fontane was NOT based on Frank Sinatra. However, everyone assumed that it was, and Sinatra was furious; when he met Puzo at a restaurant he screamed vulgar terms and threats at Puzo. Sinatra was also vehemently opposed to the film. Due to this backlash, Fontane's role in the film was scaled down to a couple of scenes.
George Lucas put together the "Mattress Sequence" (the montage of crime scene photos and headlines about the war between the five families) as a favor to Francis Ford Coppola for helping him fund American Graffiti (1973). He asked not to be credited.
James Caan originally heard the phrase "bada-bing!" from his acquaintance, the real-life mobster Carmine Persico, and improvised its use in the film.