The actor who played Luca Brasi in the Godfather was actually a member of the Colombo crime family sent to monitor the set. Coppola kept the cameras rolling as the actor nervously practiced his lines for the scene, scared that he was going to mess up the big Hollywood movie. He later edited that footage into the film so in-universe the actor’s character was nervously practicing what he was going to say to the intimidating mafioso Don Corleone.
The “real Mafia” did not allow the producers of “The Godfather” to use the word Mafia in the movie. They even sent “family” members to the set to monitor filming. That’s how producers discovered Lenny Montana aka “Luca Brasi”.