The funniest play I ever saw - from the stage - was Shaw's Caesar and Cleopatra. Not a hoot-fest, you say, DR's. True. But I was doing the production at the Shaw Festival and the company was nearing the end of the interminable Canadian run before going on tour. We were all a bit hysterical and exhausted. So in the final scene, as Caesar is about to sail off, and we're all standing staring into the house (the house being the ocean), the actor playing Epiladorus (can't remember how to spell it) sneezed and the actor playing Caesar said, totally in character, "Bless you, my son." For some reason, I thought this was the funniest line that had ever been uttered in the history of the theater. I totally corpsed, except there was nowhere to go, nowhere to turn as I had to keep staring out at the damn ocean. These strange little strangled sounds started coming out of me and I was literally peeing my pants (panties) - which is incredibly horrible in a diaphanous Egyptian gown. Anyway, I finally had to turn around and face upstage - so while everybody else was staring out at the ocean, I was looking at god knows what behind me. I'm surprised I wasn't kicked out of Equity.
So, that DR's is the funniest theatrical experience I've ever had.