Wiseguys hanging around movie sets was rampant back then, especially when the subject matter was about them. It goes all the way back to George Raft and his obsession with hanging out with hoodlums (he had a lot in common with James Caan in that respect).

Back in the early '60s, Sinatra and Company made a fun musical called "Robin and the Seven Hoods." There are at least two bona fide killers in that film. One of them even had a small speaking role .