Monty Python and the Quest for the Holy Grail - Actor Laughing
Go to the part where they have a supposed witch. When they go into the big discussion about how to tell if she is a witch, John Cleese will take a big pause while thinking. Look at the guy next to him biting the tool he is holding.
Oh man, there are SO many instances of just this sort of thing throughout the series, barely suppressed laughter, mostly by Cleese or Palin but everybody at one time or another! These sketches come to mind: Palin's inept Swiss-watch smuggling, Palin in the pet shop offering to convert a terrier into a cat/bird/fish, Cleese & Chapman listening to "The Death of Mary, Queen of Scots" on the radio, Gilliam as Cardinal Fang reading from the list of charges, etc.....
I agree with everyone about this comment. My brother and I have watched this movie together countless numbers of times, and each time when we have seen that part, it has simply been stated "Watch Eric Idle bite his scythe". Either way, you can easily tell that he is not laughing, rather biting it, or thinking hard. People often do funny things when immersed in thought. But even if he WAS laughing, it's true, if you were the Pythons, would you be able to keep a straight face through even that little bit of comedy?
In the commentary track of the newly released "Special Edition" DVD, both John Cleese and Eric Idle point out that Eric is in fact biting the scythe to keep from laughing. John took a VERY long pause before "... 'cause they're made of wood?", and Eric couldn't take it anymore, so instead of ruining the take, he bite the scythe to look like he's thinking, but a rewind/keen eye can pick it out (though the commentary DID help).
Idle is definitely laughing. In the DVD commentary one of the Terrys, John Cleese, and Eric Idle all comment on it. Apparently, Cleese took a huge pause when answering, and Idle didn't want to ruin the scene.