Are you certain that they weren’t credited in the rolling credits going 895 mph?

Robin Williams played the King of the Moon. The credits list “Ray D. Tutto”. This is the English transliteration of the Italian phrase “Re di Tutto”, which means “King of Everything”, which was how the King of the Moon introduces himself to the Baron.

I’ve heard various reasons for an actor not being listed in the credits, from hating the final project so much that he wants his name disassociated from it, to performing as a favor for a friend at below scale wages which might violate a contract with another studio.

Generally actors go uncredited by choice but occasionally it is just an error in the titles. IfI recall correctly Kathleen Turner is uncredited as the voice of Jessica Rabbit in “Who Framed Roger Rabbit” due to a simple oversight.

Robin Williams also did Shakes The Clown, and was credited as “Marty Fromage”.
@Blueroses got it right.

Ooooo, I have another one; he was the guy in the stockroom in a cameo role in Dead Again. I never forget the scene. He offered Kenneth Branagh a smoke. Brangh said no, that he wasn’t a smoker. Williams said that he was too a smoker, but that he chose not to smoke. He said “Figure out what you are and_be_ it.” He is not listed anywhere in the credits. How odd.

Actors make unbilled cameos all the time. It helps ratchet up interest in both them and the movie.

I don’t think Alfred Hitchcock was ever identified in the cameo bits he did in his movies.

@anartist I don’t think it was necessary. He had the leading credit and the inside joke was watching for his cameo. Kind of like the “Nina” in Hirschfeld’s New Yorker cartoons.

Another uncredited appearance I thought of- Willem Defoe in Cry Baby.