Trivia: The term "godfather" was never actually used by the actual mob, author Mario Puzo just made it up. Yet after the book and film came out people started assuming it was, so it started appearing in news reports. And it is now actually used by the mob.
Trivia: Al Pacino's real-life grandfather was born in Corleone, Sicily, the same town that the fictional Vito Corleone, Michael's father, was born in and from where he adopted the family surname. As a child, Pacino's family called him "Sonny." That is the same nickname his on-screen brother, played by James Caan, was called. Revealed by Al Pacino on The Actor's Studio.
Trivia: In the scene where Sonny is killed by the men with the Tommy Guns, James Caan was very apprehensive about how many squibs he was wearing (147, a record number at that time, and therefore very dangerous). He only did that scene because he didn't want to lose face in front of the female crew members.
Trivia: Woltz's reaction to seeing a horses head on his pillow was real. The actor was unaware of what was to unfold in the scene, making it genuinely shocking.