Ever have a spurt of inspiration reveal a new dimension of your plot or story? It could be a brave new adventure or a step off a cliff into the abyss. Here's a second post on how to vet inspiration — for plot twists and subplots.
As is true for character-related inspiration, it's common to uncover new subplots — as well as plot threads, changes and twists — in draft two. In draft one, you're still getting to know the characters and the story they tell, so many problems will resolve by the end of the draft, or in the second. But what about those persistent problems and your idea for a twist or subplot? Disaster of disasters — you may even decide to change the plot entirely halfway through or earlier. How do you know what will work and what won't?
Many writers can follow a plot through its meanderings without an outline. Some can follow more than one. But if your story is complex — and even if it's not — you may want to record your idea in an outline or a tree diagram. It's the best way to see how the idea will "flesh out," literally. Plot shifts often arise from changes in characters. They mature faster, into different people or in different directions than you thought. Now your plot can't contain them. It's too thin, not compelling enough to fit who your people are becoming. And the more I learn, the more I believe characters drive plot, or should, rather than the reverse.
Here's an example. A woman returns to Barcelona for business and family obligations. She dreads the trip because her grandfather, with whom she was close as a child, has dementia. On the surface, the story is about the woman confronting the reality that life in the land of her youth is no longer what it was, a la You Can't Go Home Again. However, when the woman boards a train to the Mediterranean, she meets a young artist who reminds her of another creative type she met years earlier. Does she engage in conversation with this younger man and leave it at that? Probably not. Tracing her decision to have an affair, presumably to avoid the sadness of her family situation, could bring her full circle to realize she can't expect anyone to "create" life for her, that her life is her responsibility, as are her choices and their consequences. A difficult lesson, but one that makes the outcome more valuable because of the cost involved. Here, it's important to understand why the woman makes one choice instead of another, even if the writer doesn't include all the details.