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Think of setup as a promise made and payofs as a promise fulfilled. When you set something up, you create an expectation in the reader that it'll be paid off later. In Thelma and Louise, as Thelma is packing, she throws in her gun on a whim, even though she has never used it and is afraid of it. When we see that, it propels us into the future. We know that gun is going to be used and wait for it to happen. If it doesn't, we'll feel cheated.
Every story element needs to be set up: actions, character traits, character transformations, events, turning points, the ending. In The Pursuit of Happyness, set in the 1980s, we see the hero playing with a Rubik's cube, and we hear a report on TV about the fad it's set off (setup). In a later scene, the hero attempts to talk himself into a prestigious training program for stockbrokers, but the broker he's trying to impress is too wrapped up in the Rubik's cube to listen. The hero manages to complete the puzzle, thereby proving his smarts to the stockbroker (payoff).
When you set something up, it needs to evolve organically and naturally. The point is to tease without blatantly manipulating. It can't scream, "Hey, you! Look here!" Make the intervening stuff between the setup and the payoff pertinent to the story and interesting in its own right. Then deliver the payoff. Work on getting the timing right, which will vary from scene to scene and story to story. Don't leave people hanging too long, but don't close the gap too quickly either.