One of the interesting things about writing fiction is that even when you think you know the story you’re writing, you can still be surprised by what ends up on paper. (Or represented electronically on screen.)
In the chapter I just finished, for example, the introduction of the character Kairith was planned as part of the plot. I knew who he was and what he had to do in order to advance the storyline. But I didn’t know what he was like until I wrote him.