I've only just recently realized that the reason why I enjoy writing smaller characters in fandoms is the same reason why I enjoy creating OCs: all. that. backstory.

Yep, pretty much. Nothing makes me happier than backstory. I mean, I have fully fleshed out Holmes cousins, and I've even mentioned them anywhere. I read a good quote the other day, which of course I can't find, now, but it basically was 'you can't write a character until you know where they have been and done, and how they got to this moment in their lives', and that's sort of my philosophy, too.