And James Caan’s (official) version:
It’s the ba-da-bing, don’t you know. And the fact that Caan’s Sonny knows from whence he speaks, while De Niro’s is a street punk playing at being a made guy.
Of course, De Niro would go on to give one of the greatest performances in American film history in The Godfather Part 2, and communicate a sensitivity and subtle intelligence you would not have thought possible from that goofy goombah bit.
Either those two years matured De Niro as an actor in a remarkable way, or Francis Ford Coppola knew exactly how to mold that performance with the clay he was given.