Yeah, that's where writing gets fun (on another level) because each and every character you write is a potential main protagonist or main antagonist.  Just because they're in one story doesn't mean they're not going to come out and tap you on the shoulder and say, metaphorically, of course: "Hey you!  Write my story now."  If you have 5 characters in a story, that's 5 stories you can potentially write.  I love it when things like that happen.