Sometimes the enjoyment for an audience member is from an expectation fulfilled, this is why that Mitchell and Webb sketch is so funny, we know where it's going, we know quite early on how the sketch is going to end but the enjoyment is watching the journey through to it's conclusion.
The big laugh at the end of the sketch is almost one of relief because we, the audience, have got the resolution that we wanted and were expecting.
Thanks for the feedback on the Columbo sketch, guys. I didn't get to see it performed and it's hard when you're writing something like that to judge when the audience is going to pick up on what's going to happen and how long you've then got to pay it off before they lose interest ... although thinking about it now it might have been more interesting to not deliver and just have him go 'it doesn't matter' or something when he turns at the door.
If you get round to reading this, do either of you happen to remember if they stated the sketch off when he comes into the shop, or when he's already paying for the stuff?