I write short fiction in many genres, and I often know before I commit the first word to the page in which genre a new story will fit. On the few occasions when I don't know before I begin writing, I know before I've completed the first page. Every so often, though, a story takes a left turn on me.
I often write just enough of a story--a few hundred to a few thousand words--to know where a story is headed, and then set it aside if it is seasonally inappropriate or if there is no market for it. An example would be the setting aside of a Christmas story in January, when it is far too soon to submit, in favor of working on an April Fool's Day story or a Mother's Day story.
When I return to work on the set-aside stories, I can usually pick up where I left off and finish them as originally intended, and that was my intent with a story I recently finished.
I had written the first 1,500 words of a confession about a woman who discovers that her deceased father had a long-term relationship with a woman who wasn't her mother, and my intent was to have the first-person narrator learn that she had a half-sister that she'd never known.
When I resumed work on the partially written story, I realized the narrator's father's secret was much deeper and darker than an affair and a second child. As I continued writing, my confession became a mystery, and, by the time I finished a complete draft, I had a great story poorly told.
I had to work my way through the initial draft to ensure that all of the clues fell into place properly, that the confession style of the first third was rewritten to conform to the mystery style of the last two-thirds, and I had to research some historical information to ensure that what I claimed happened in the story either did or, at least, could have happened when I said it did.
The finished story doesn't much resemble its confession rootstock, but it does retain two key elements of confession: The story is written in first-person by a female narrator.
The story is now sitting in the electronic slush pile of a mystery magazine, and I'm eager to learn if my letting the story take a left turn was a trip worth taking.