I’ve seen him in 4 movies where he makes a cameo at the start:

• Strangers on a Train: he’s a passenger getting on the train with a guitar after Guy Haines (Farley Granger) steps out
• The Wrong Man: during the intro before the main movie starts, he discloses that this movie is based on true events
• North by Northwest: he’s a commuter whom missed the bus at the last minute upon the opening credits finishing
• The Birds: he’s walks out of the pet store with dogs on a leash right after Melanie Daniels (Tippi Hedren) enters