Side Characters

I'll be the first to admit that I'm not an expert in creating side-characters. I think those that I've created have worked well in my stories and part of that comes from a bit of a sixth sense. So, I wouldn't take any of this advice as anything but the ramblings of a person trying to figure it out on their own as well.

But, like every part of writing, creating a good side character is about asking a lot of questions.

What would be interesting? What purpose would they fill? What would their life...