Set up and Payoff

Movies are full of them... the setup and the payoff.

It's the part in a film that introduces a person (or object or piece of information) that seems to have little significance to the story. The audience shouldn't see it coming, yet it is significant enough so that the audience will remember it. That's the setup.

The payoff happens when that seemingly insignificant information becomes a very important plot point.

In The Wizard of Oz (1939), think of how the ruby slippers are set up as being very important. The reason originally given by Glinda to Dorothy is that if she takes the slippers off, she'll "be at the mercy of the Wicked Witch of the West." However, by the end of movie, Dorothy finds that these same ruby slippers are the key to getting her back to Kansas! There's your payoff.

Another classic case in point: One Flew Over the Cuckoo's Nest (1975). Remember the scene when Jack Nicholson is showing off to the other guys in the psychiatric institute? He boasts that he can lift this huge marble water fountain. Of course, he can't; it's too heavy. On first viewing, it just seems like a great little scene meant to show the sheer audacity of the Jack Nicholson character. However, it's really meant to be a setup.

Now to the very end of the movie, when the big Native American patient escapes from the asylum... How? By lifting that same marble water fountain (the one that wouldn't budge for Jack Nicholson). But this guy not only can lift the thing, he throws it through a window and is able to escape from the asylum.

What if this final scene hadn't been set up with that earlier scene? Well, we wouldn't have been amazed at the sheer strength of the guy. (When Jack tried, we realized how heavy the thing was.)

For more on setups and payoffs, try reading these:
Plants and Payoffs
Set Up and Payoff