Many years before I began writing Goddess, I read an article where an author described coming up with the plot of his latest book. He explained that he was going for something different, but he couldn't "make the characters do what he wanted them to do."
"How silly," I thought. "You're writing them. You can make them do whatever you want."
It was only after drafting a few chapters of Goddess that I realized how truly right he was. I don't know how it works, but your characters *can* fight back, and they can determine their own opinions and their own course. For example, I had planned for two characters to be romantically involved, and then it turned out they just had no chemistry together! I had also planned for a character to be selfish and spoiled, and she fought me every step of the way to reveal her true personality.
I'm sure there is some kind of scientific explanation, but I'm not sure I want to know. That's the joy of writing fiction - sometimes you control the page, and sometimes the page controls you.