A joke is a lot like a movie with a twist ending. Think about the first time you saw The Usual Suspects, and how shocked you were when you found out who Keyser Soze was. Yet when you watch the movie again, it all makes sense. The movie was giving you clues the entire time, but not enough to give it all away.
A joke works the same way. If, while you were telling the joke, you mentioned that the man was lying in seats just beneath the balcony, the audience will probably be able to guess what happened, and the surprise will be ruined. It may seem like cheating to avoid mentioning the balcony until the punch line. It's not. A lot of movie theaters have balconies, and the revelation that this theater is one of them provides the surprise.The punch line to this joke works because it instantly switches the perspective of the audience. Like the usher, the audience believes that the man is rude, or creepy, and that's why he's taking up several seats. But when we hear the man's perspective, an entirely new picture is revealed.
Timing Is Everything
Here's where things get really tricky: timing. There's a reason the phrase "punch line" contains the word "punch." There's a reason the phrase punch line includes the word punch. Like a prizefighter, a joke teller should know exactly when to deliver the coup de tat, closing the joke at just the right moment.
Determining the perfect time to spill the punch line isn't easy, and worse, timing will differ from joke to joke. This takes practice. There are clues in every joke, however.
Let's take another look at the example. Just before the punch line, there's the phrase "The man looked up helplessly and said..." This brief bit is immensely important. Keep in mind, then, that it's not your job to let the audience know the punch line is coming. The joke should take care of that on its own. That's what's happening in this line. No need to overdo it.