I can’t remember what the first piece of fiction was that I ever wrote, but even without being able to recall that lost and lonely tale, I already know how the characters in the story were created. They created themselves.
Different writers of fiction have different approaches to the craft. Different ways of creating a storyline, different ways of imagining characters and making those characters come alive. For me, the process has always been like transcribing what I see unfolding on the big screen inside my mind. If it wasn’t happening inside my own head of which I (supposedly) am the owner, it would be fair to call my style of writing and character creation a form of theft. Because rarely do I know very far in advance what a character will look like, how he/she/it will sound, or what they will do in a given situation until it happens. It’s as if the minute my fingers touch the keys, the bones arise and assume flesh before my very eyes. Sometimes I’m more fascinated by these guys than those who read the story later on. I just can’t figure out where they came from.
This may not be much of an answer, except possibly in the lesson that I have learned over the many years that I have been writing stories; let it happen. The minute you think you know better what the story is than the story itself, or who a particular character is when they keep trying to introduce themselves to your deaf ears, is the minute you lose your creativity. And the minute you lose that creativity is the minute you lose your characters and your stories.
Let it happen.