"....so what's your answer gonna be pop?" he mumbles through the peanuts.

I've mentioned this one in the past - toward the end of the Corleone/Sollozzo meeting in Vito's office, Vito gets up and pours another shot of annisette for Sollozzo. As he pulls away he apparently let's a drop fall on Sollozzo's pants, which he quickly brushes off. Obviously an ad lib, but it shows how Brando stayed in character.