My short stories usually begin with an idea that gets expanded into an outline, and then written. While this works fairly well most of the time, sometimes a story will go off into a different direction than I had initially intended.
A fairly straightforward story might jet off into the Twilight Zone or one that was supposed to be funny will become sad. It is often at these times that I will become stuck as I think over just which direction I want to take the story in. Once when this happened, I wrote two stories: one that went off in a silly direction and another one that played out more straightforward, because straightforward had been what I had originally envisioned. Sometimes I will just keep writing even if the story isn’t going in the direction I want it to, just to see what comes out of it. Sometimes it works, sometimes it doesn’t.
No matter how much you plan, you really never know where a story will go until you actually start writing it, because as crazy as it sounds, stories write themselves.