Have my permission. List of people that are free to use my characters at all times and without permission
Keep them IN CHARACTER. If you have Cass going around willy nilly killing or Cyanide flirting with men, I'm gonna be irritated.
Don't abuse them in order to progress yourself. I'm all for selling in order to progress a story or build somebody else, but not at the stake of my characters looking like crap, especially when you're the one NPCing them.
That's pretty much the basic guidelines. There are a lot of specifics for each character that I'll get into later.