When writing comedy dialogue, an 'easy' way to get a quick laugh is to have a character make an instant U-turn (I'm sure there is a name for this process, but I can't think of it).
I'd say the technique is used in US sitcoms more than British ones, so to illustrate what I mean here's an example from Frasier (I made this one up but similar exchanges abound between the Crane brothers).
Niles asks Frasier if he would like to accompany him downtown to see a performance of La Triviata.
Frasier: Certainly not. That company had its behind universally paddled in the notices.
Martin: Good. You can stay home with me. There's a sports bloopers double-bill on at nine.
Frasier: (to Niles) How much are the tickets?
I'm not suggesting for a moment that Frasier scripts are weak, but in the above exchange, I would have seen the punchline coming as soon as Martin began to speak - mainly because this technique is used so much in that show, the audience can see it coming.
Thoughts?