Not sure if this is true, but I heard that Tom Cruise is one of the first guys shot dead by Emilio Estevez (the mighty duck man) in Young Guns when he pops out of the box at the very end.
Kurt Vonnegutt in "Back to School," hilarious.
But Alfred Hitchcock might be the correct answer. Always appeared in some bit part, usually a very humorous appearance.
Lots of good ones listed. I didnt really think about it till someone else mentioned it but Chuck Norris in Dodgeball was awesome. I dont think anyone expected that after going through the judges at the end.