The Surprising Truth About MRUs: Where They Work—and Where They Don’t
A “motivation” is the term Dwight Swain uses to refer to anything that happens external to your Point-of-View character.
A “reaction” is the term Dwight Swain uses to refer to anything that the POV character does.
You might say that this is confusing terminology, and I whole-heartedly agree with you. But Dwight Swain was the master, and he created the language, so we’ll use his terms because his ideas are really, really good.
Here’s an example of a “motivation,” taken from my perfect-scene article:


