They totally take over - the story, my life, my sanity....XD All together not a good sign. I tend to write out my main characters first - I've always stuck with four, I'm not sure why - and I find them a setting, occasionally exceptionally strange ones so I can crack (a feint at some kind of literary intelligence). And then, I just talk to them in which case hearing strange people's voices in your head is no longer a sign of schizophrenia and they tell me where they want to go. I can't write them when they don't talk to me, and occasionally they tell me secrets from their past that completely screws up my plot and I have to go back and edit some 50 odd pages...... And despairingly my supporting characters aren't better since I make them in relation to the main characters in that they fill up a personality/spiritual/whatever lack.

So in conclusion, I have no plot. I have a bunch of interesting people who're going through their life and forcing me to write about it. Would be great if these people weren't just fragments of my imagination, but I guess you can't have everything. :D