A standup comedian doing his/her set is usually funnier than when he/she acts in a comedy movie

For instance, I've only seen 2 of Robin Williams' stand-up performances and they were both hysterical, but I can say the same thing about only 1 of his comedy movies (Mrs. Doubtfire).

Same with Jim Carrey. The only movie of his that I found to be hilarious was Dumb & Dumber, but the 2 standup sets of his that I saw were both excellent.

Rodney Dangerfield is in the pantheon of standup comedy, but he had, at best, 1 sidesplitting movie (Back to School).