I was just re-watching "The One About Mac And C.H.E.E.S.E." (the one that should have been titled "The One That's Really A Clip Show) and I have the following question regarding this and other clip shows(ie, shows that are comprised mainly of old clips from previous episodes):

Are the new segments (the parts that set up the clips) shot live in front of the studio audience? And if that's the case, is the studio audience then shown the clips that we see in the finished episode?

And also: Do the writers write out the old scenes from the clips from previous episodes or do they just leave that section of the script blank?