A lot of details in the films came from the real crime families of the time, including even Marlon Brando’s voice. He based it on the real mob boss Frank Costello, who had recordings taken of him during one of his trial hearings.
The thick, powerful, mumbling method of speech inspired Brando as he was conducting his research, and when he went in for his audition he stuffed cotton balls into his mouth to produce the muffled voice. Of course, Coppola and the producers loved it, and even now we can all hear it in our heads.
Got in Through Other Ways
Not everybody in "The Godfather" was an experienced actor. Some were even parts of the real Mafia. Most of them didn't do much beyond set dressing, but Gianni Russo, who played Carlo Rizzi, was originally a mob associate to Mafia boss Frank Costello. When he heard about the film, he used his organized crime connections to secure a role in the film.
The role was small, but he was still present – playing Connie's husband, who regularly abuses and cheats on her. While Vito does nothing – Italian tradition forbids parents meddling in children's marriages – but Connie's older brother Sonny has no such hang-ups.
A Real Threat
As this was Gianni Russo's first film and he didn't have any acting experience. He was flying by the seat of his pants when it came to his character, which was frowned upon by Marlon Brando. That changed quickly when Russo approached Brando and threatened him.
Brando had no idea this was a real threat but thought the way Russo delivered the threat was so fantastic his opinion of the man changed in a heartbeat. That's Brando for you. We don't know what the threat was, but Brando laughing it off is quintessential Marlon.
Into the Thousands
"The Godfather" had an incredible amount of extras signed on to help, and there are few scenes that reach a monumental number. For instance, during the scene where Sonny brutalizes Carlo used over seven hundred extras while filming it.
The wedding scene tops even that, using more than seven-hundred and fifty. The number of extras gives the film a huge, real effect, and if you've ever been part of an Italian wedding, you'll find it runs pretty close to real life. Some of the guests don't even know the bride or groom!
Packing on the Pounds
Marlon Brando was a pretty big guy during the filming of "The Godfather". Even so, he decided to pull a few jokes on some of the other actors. When Don Corleone has to be carried up to his bedroom coming home from the hospital, the hectic scene is full of people trying to get out of the way and others trying to carry him up a set of stairs.
Marlon Brando decided he needed more weight in his bed, and packed it full of additional weights, just to give the actors who had to carry him a harder time.