For the scene where Clemenza is cooking, Francis Ford Coppola originally wrote in the script, "Clemenza browns some sausage". Upon seeing this, Mario Puzo crossed out "browns" and replaced it with "fries", writing in the margin, "Gangsters don't brown."
Little known fact: very few people wanted Al Pacino for Michael. Pretty much no one, other than Coppola, wanted him. But when Coppola pictured the scenes of Michael wandering the Sicilian countryside with his bodyguards, he saw Pacino’s face. Coppola eventually wore the execs down.