 Hal Needham (qv), on the first films commentary, talked about how  Frank Sinatra (qv) showed up very early on the set of this film to  get his parts shoot, and then left before the other actors even  showed up. If you watch closely during the office scene, Sinatra is  never on film with the other actors. A few times his back is shown  with the other characters facing him, but this is a double.