Note how Tom Hagen signals Clemenza when he loosens his tie.
Actually, I missed that, or at least didn’t interpret it as a “go” signal.
What’s really interesting is that Russo the person had, or claims he has, mob experience. The Carlo of the movie looked like a minor character actor. If Russo isn’t a complete fabricator, then he’s a good actor for going down-role for the Carlo character.
As an aside, Howie Carr wrote that the guy who played Moe Greene was in real life associated with the White Bulger mobsters of Boston.