Sure, Brando and Pacino will play mafia chieftans in The Godfather. But those are cute Mafia chieftains. They're only warring on bad Mafia guys and crooked cops; they're only trying to hold the family business together. Try asking a major star to play a real Mafia head, a man who makes a living off whores and child pornography, heroin and blood; sorry folks, those parts go to the character actors or the has-beens. Or actors on the come who haven't achieved star status.
W. Goldman, Adventures in the screen trade (1983), 37