Much of the action in my story is motivated by various characters' responses to a character who, when the story begins, has just left the rest of the characters in a way that guarantees they're never going to see her again. (They know she isn't dead, but she's physically unable to come back even if she changes her mind and wants to.) The two questions I've been struggling with, the questions that are going to be the central points of the story, are a. why did she leave? and b. why are the other characters' feelings about her, and their reactions to her departure, so ambivalent?
I've been pretty sure for a while now that the answer to both questions is the same: This character did something completely, unquestionably, unforgiveable. I knew that either she'd done it for a reason which the rest of the characters could forgive (i.e. they could accept the intent behind the action, but not the action itself) or that the action had had a result that the rest of the characters considered favorable (i.e. they were torn between accepting that result and rejecting the way the result was achieved). As I said, there had to be reasons that the other characters' reactions to her were ambivalent, and remained ambivalent throughout the story.
That was kind of where I stopped. This is absolutely characteristic of my fiction-writing process, by the way. I think of something, a character, a backstory, a situation, and I let it play itself out (usually in my head, sometimes written down a bit) until it reaches the point where I'd be forcing it if I made it develop any further.
At that point, I go over all the other characters/situations/bits of stories that I've written previously, for other things, and I see whether any of those pieces hook neatly onto the new piece I've created. (Or, all right, whether they could be made to hook together if I altered them both a bit.) If none of them do, then I put the new piece back into the general files with all the other pieces, and wait until I think of something else.
Periodically, I read through all the files (some of which are, again, in my head rather than written down) and see whether any of them speak to me in a way they didn't before. Maybe I've had experiences that make these pieces ring more true to me than they did when I wrote them. Maybe I've met people on whom the faint sketches of characters can be based to flesh them out and make them real. Maybe inspiration will whack me over the head if I read the files in this order, instead of that order. Anyways.
So I was trying to think of what this woman in my current not-story could possibly have done, to cause these reactions. And something from an old file jumped out at me--ten years old, at least, and existing only in my head because I didn't quite dare write it down back then.
It's one thing to think of the most awful act a character can commit, and then plot out how other characters would react to it, while still writing that character as a sympathetic character, someone the reader is supposed to like.
It's another thing entirely when you realize that the perfect act for a character to commit, the thing that would not only make sense of all the half-formed story bits you've been collecting but would actually make it a stronger story than you anticipated...is an act that you, the person writing this story, find so repulsive, so utterly unforgiveable, that you have no idea how you can create a character who is sympathetic yet could commit this act.
This is the right thing for this story. It's a good idea. It's almost, almost, a plot-crystallizing idea, and goodness knows I don't get many of those. I'm just not really a good enough writer to write it. Yet.