As a writer, often comes the struggle related to the “egg or hen” paradox:
What comes first, the plot, or the characters?
I think to each author their own technique in that regard. Personally, it’s a mix of both in my work process: Most of what I write is the result of cramming ideas into my notebook, which I then polish and insert into the narrative. However, I tend to be very character-oriented, and spend a lot of time fleshing out the story’s actors.