The man who stops to greet Vito in this scene from The Godfather Part II was actually just an extra named Carmelo Russo.
He was supposed to walk past Robert De Niro without saying anything, but during the take he decided to stop, shake his hand and greet him.
Coppola reportedly wasn’t happy that Russo had gone off script, but De Niro liked what it added to the scene. Vito had become such a respected figure in the neighborhood that even someone passing him in the street would stop to pay their respects.
The unscripted moment ended up staying in the film.