Just in case you weren't aware, Michael Swanwick is a genius and writes science fiction. These are his words:
"A story requires at least three characters. So the triangle a useful tool when analyzing why a story-in-progress doesn’t work. Make a diagram of all the characters and who they interact with. Look for triangles. If there are none, then you’ve identified the problem:
"A protagonist needs to be pulled in two different directions, so there can be a resolution that is a synthesis of interpersonal forces. A protagonist and an antagonist (who would be represented by two dots connected by a line) don’t enact a story – they’re just playing tug-of-war. Which is no more a story than is a football game.
"So a man falls in love with a woman. Either it takes or it doesn’t. No story. A woman has to choose between two men. This might be a story. Draw the triangle.
"There’s a line from her to Man A and another to Man B. But is there a line between the two men? What is their relationship to each other? Usually when such a story isn’t working – when it doesn’t feel like a story – it’s because the two men have no direct relationship with each other, but only interact through the woman. Ask yourself how you can make their relationship interesting. Are they best friends? Father and son? Astronauts competing for a place on the first rocket to Mars?
"The insight can be extended to ask related questions. Is the relationship on one side of the triangle significantly weaker than the other two? Are there more than one triangle in the story, and if not should there be?
"As you can see, the utility of this is extremely dependent on the specifics of the story in question. The one universal that I insist upon is that the triangle is descriptive rather than prescriptive. We can all think of perfectly valid stories that don’t have character triangles in them.
"And it does no good to start with a triangle. Let your story find its natural shape. If you get stuck, diagram it out and look for the triangles. If the story doesn’t get stuck, don’t give it a second thought."