I love The Godfather trilogy; they are perfectly executed films. Not many people care for the third one, but in all seriousness it's a great end to the trilogy. Now can you imagine anyone else other than Al Pacino as Michael, James Caan as Sonny, and Marlon Brando as Vito Corleone? Because it could have happened.
Some of the auditions you'll see in the video below, inlcuding footage of Pacino, Martin Sheen and Diane Keaton. I've also included a documentary on the film, called The Masterpiece That Almost Wasn't, that goes into some interesting details about how the movie almost never got made, and reveals a lot of other actors that could have ended up in these iconic roles. If you're a fan of The Godfather, then you need to watch these!