There are two kinds of writers.
One kind thinks about the story they are going to write. They think and think and think about the story, then one day when they are done thinking, done imagining the story, they go and they write it all down in one feverish rush. This kind of writer is very, very rare. They may even be the stuff of myth and movies. They are the mad scientists of literature.
The other kind of writer has an idea about a story – a character, a plot point or two, a place, a situation – and gets a draft of the idea down as soon as they can. They write a reckless and awful first exploratory draft to see if the story has legs, if the idea is going to sprout. This is almost always clear to the writer in this first draft and is always clear by the second. There is either life in the idea or there isn’t. It is either on or it is off. If there is life then the writer reworks that draft over and over until they are convinced the story is fully wrought.
I am this second kind of writer. I learn what the story is going to be as I write it. That original seed of an idea is only a place to start. The story is created in the writing of it. This revealing of the story as it is written is called progressive elaboration*. As drafts of the story are written, the story unfolds and the facets of it develop. I believe that all novels are like this. It is certainly my experience that all short stories are. With each draft you find there are aspects of the story to be fleshed out, other aspects to be left less explicit, for the reader to imagine.
It is in the progress of making the story that we learn what it will be and how it will be told.
*This is a term that is used in project management – discovery of the work to be done as it is planned. I am repurposing the term here, as it applies perfectly to the making of fiction as well.