A “plant” is a prop, line of dialogue, activity, or thematic beat that is included in an early scene, and the “payoff” comes when it reappears in a later scene, in a new context and infused with a very different meaning.
Example of an OBJECT plant and pay-off:
In Sideways, a wine bottle dated back to 1961 is introduced as one of Miles’ most prized possessions. He looks forward to drinking this wine at a special occasion. Later in the movie, after he has been to his ex-wife’s wedding and realized she will not get back with him; Miles ends up drinking this prized wine in a McDonalds by himself, from a cheap plastic cup.
Example of a DIALOGUE plant and pay-off:
In 300, before raping Queen Gorgo, Theron says: “This will not be over quickly. You will not enjoy this. I am not your King.” Near the end of the movie, Gorgo speaks the same words back to him as she thrusts a knife into his chest: “This will not be over quickly. You will not enjoy this. I am not your Queen.”
Example of an ACTIVITY plant and pay-off:
In The Silver Linings Playbook, Tiffany run safter Pat and catches up to him the first time they run together. At the end of the movie, after the dance competition, the situation is reversed: This time Pat runs after Tiffany and catches up to her.
Example of plant and pay-off tied to the THEME:
One of my favorite examples of "plant and payoff" occurs in The Social Network. In an early scene, Sean Parker suggests to Mark Zuckerberg that he tell potential investors and partners, "I'm CEO ... bitch!" In the scene, this is a funny line, one that stirs Mark's ambition and the audience’s excitement about what Facebook will become.
It’s a line that means, “I’m a badass! This is fun! We’re in this together!”
It is a plant.
Later in the movie, after his best friend, Eduardo Saverin, is pushed out of the company (and betrayed by Mark) and Sean is arrested for drug possession, Mark opens a box of business cards, which read "I'm CEO ... bitch." The meaning of the words has entirely changed. They are now an emblem of the price of ambition and success. For both the audience and the protagonist, the words feel like a slug in the stomach.
Now the same line means, “I’m an asshole. This is no fun. I’m all alone.”
The key part of any plant and payoff is this change in meaning.
In this way, Plant/Payoff is a kind of metaphor. "I'm CEO ... bitch" comes to encapsulate the movie's underlying themes juxtaposing Zuckerberg’s single-minded ambition with the longing for human connection.
I like to differentiate Plant and Payoff from straightforward EXPOSITION. We may see a baseball mitt early in a film, and then see it in a later scene when the hero plays baseball. But the meaning of the mitt hasn’t changed, so it’s not really a payoff. The placement of the mitt in an early scene is just a piece of information we need to know to understand later events in the story - i.e. exposition.