I write short stories and novels. I do not write episodic productions that are aimed at being bought by Netflix.
I have added characters as needed in the middle of writing a story. They are usually based on a person who actually exists, which means not completely on the fly.
I have a rough outline for the novel before I start, but not for a short story. As I continue to write I will start putting more in-depth information in the upcoming chapter outlines because I don't want to leave out a necessary part of solving a mystery or other challenge for the protagonist.
I don't have a seasons worth of stories. I have a novel with chapters that is complete before it is ever submitted for publication.
You didn't ask this question but just in case you want to know—writing a syndicated novels for which a chapter is published every week really requires a detailed outline or the whole story actually being written in rough draft before the first chapter is published.