Characters: Michael Corleone and Kay Adams
Movie: The Godfather (1972), The Godfather: Part II (1972), The Godfather: Part III (1990)

Diane Keaton is another repeat offender when it comes to dating her co-stars. This time around, it's Al Pacino. Keaton as Kay in The Godfather never seemed quite dangerous or wild (or criminal) enough to be married to Pacino's Michael.

It appears that Keaton off-screen was not quite the woman Pacino wanted to marry either. The couple's off-again-on-again relationship lasted through all three parts of the trilogy, and ended, according to some, when she made an ultimatum: marry or break-up. Keaton's memoir highlights another potential cause: her recurring bulimia, which according to her, caused the end of her relationship with Woody Allen.