I've had this happen before, though not with fanfiction, unfortunately. But, instead with my roleplay characters. I imagine them and write their application to be a certain way, but then they end up being these ultimate soft people that I just bang my head and go, 'how the heck did that happen?' and yeahh. I'm in the middle of writing a fic though that was going to go strictly to the happenings of the series (seeing as how it's about an OC whose friends with Harry) but I've decided to just push my books aside and see what happens. My character, Maddie, hasn't gone crazy on me yet, but I have a feeling she's going to now that I've pushed the book aside. Oh well. I'll see what happens.