Character Takeover Syndrome....

Writers, I'm sure you know what I'm talking about. When the character in your story just completely grabs a hold of you and refuses to let go until you've gotten whatever they need to say out of their system? I call it Character Takeover Syndrome. I'm sure I'm not the first to give it a name. But I am probably the first to think about it from a medical point of view. Can you imagine the commercial they'd make if they came up with a prescription drug for it? "It might be multiple personalities...OR it could be Character Takeover Syndrome, also known as CTS..."
I'm weird, so it makes sense to me. Especially right now. I've been working on a WIP, where the main character is completely kicking my butt. I need a tape recorder, because Aurora (the character) is completely a diva. You know there's always that actress that everyone calls a bitch behind her back? Say hello to Aurora.
She annoys me so, and yet, she wants me to make her the sweet little girl-next-door type? It's extremely hard to do, especially since I really haven't seen that side of her. I only know the "You're not doing it right!" side of her. I've actually started working on a tell-all by the diva, since she's always rambling. I figure, why not?
Speaking of which...I need to get back to that. I can hear her now. "You're not writing!!"

Has anyone else had a character like this? So possessive that they almost DO become your second personality?