When the audience has a laugh leader.

Mark Liberman at Language Log complains about an "inappropriate" laugh leader in an audience at a Chekhov play:

There was someone in the back of the theater with a  loud and infectious laugh,  who didn't laugh at any of the obviously funny lines, but instead laughed — maybe a hundred times — at a selection of lines that is not easy to characterize....

The laugher's interventions mostly seemed to me to be points where a character changed the subject, or said something that was unexpected in the context of the previous discourse, or said or did something awkward or socially uneasy....

But there were other theories.  One person thought that the laugher might have been a friend of a couple of the actors, who reacted whenever one of them entered the on-stage conversation. Another theory was that the laugher was reacting when the actors made certain expressive faces. These are obviously overlapping theories, and many others might be devised as well.

In the comments, Richard Bell said:

Is it possible this laugher was the one person who best understood and reacted to Chekhov's special comic gift? What you have suggested is, in fact, a pretty good description of Chekhovian comedy. His characters don't really listen to each other. Someone once said there is no dialogue in Chekhov; only interrupted monologues. They change the subject because they don't know what the subject is; they have not been listening.

Now, Liberman says the laugh leader was "loud" and goes on to describe the actors seemingly reacting negatively to the laughing. So, it seems as though the laughing was bad in some special way that makes Bell's comment an incomplete response.

But I am very interested in laugh leading. When I go to the movies or a play, I find that I myself am a laugh leader. It don't laugh loudly, but I am the first person to laugh at a lot of things, and I get other people laughing.  I'm not trying to go first. It's just that a lot of Americans — especially at high-art type movies and plays — are too polite or insecure about laughing. They'll sit there silently while all sorts of subtly funny things are happening as if they need to laugh lines and broad comedy to give them confidence. People seem to be unsure of their own perceptions or just numbed to nonobvious comedy.

A good laugh leader can lubricate the audience and intensify the pleasure of witnessing a performance with others. Of course, a bad laugh leader is a problem, especially if there are live actors on stage. But it's that fear of laughing first and laughing wrongly that holds so many people back and puts a premium on good laugh leading.