Catalyst is a term borrowed from chemistry, where it refers to any substance that precipitates a chemical reaction. In dramatic narrative, the catalyst (or agent for change, or inciting incident) is the occasion or character that sets events in motion, precipitating the dramatic action of the protagonist. Let's say that the main character learns that a former gunslinger has just ridden into town, or unexpectedly glimpses a former lover at a train station, or suddenly loses a job—any of these events could be a powerful catalyst moving the protagonist into action, into change. In a short film, we are usually introduced to the protagonist before the catalytic event occurs, so that we will have a chance to identify with him or her. Such an introduction is often, though not always, quite brief. (The first assignment at the end of Chapter 1 gave two examples of brief "treatments," up to and including possible catalysts.)
Something unexpected happens to someone—how does that person react? Does he struggle to change the way he does things, or does he instead turn away from what has happened, trying to get back to the way things had been? Does he do first one and then the other? How does any or all of this psychological activity manifest itself in the character's behavior, so that the audience has some idea of what is going on?
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