According to a Playboy interview with director Francis Ford Coppola, Frank Sinatra, despite his reported distaste for the novel and opposition to the film, had discussions with Coppola about playing the role of Don Vito Corleone himself and at one point actually offered his services. Coppola, however, was adamant in his conviction that Marlon Brando take the role instead. This would be the third time Brando performed in a part sought by Sinatra, after playing Terry Malloy in 'On the Waterfront' (1954) and Sky Masterson in Guys and Dolls (1955) .

I got this from a godfather trivia site. I did a copy and paste on the article of course.
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