Many writers will often speak of the voices in their heads. Writers, you know what I mean. Everyone else, go ahead and think I'm crazy, but I know I'm not the only one who has conversations with people who don't exist ;)
Sometimes characters become so real they take on lives of their own.
In my series, Children of the Gods, one of the characters who changed by the end of my first draft was Kara. She's a badass villain whose hard life has dictated the person she's become. She's done horrible things, but only because she's had to. In my head, she's Lyndsy Fonseca from Nikita.
Kara was never intended to be anyone more than a roadblock in Elyse's way. If you've read my series, you know she's become much more than that.
When I was writing Oppression (Book 1) I would go to sleep at night running scenes in my head, and POP, there she was. I ended up writing her into scenes she was never supposed to be in. She was such a loud voice in my head, I couldn't ignore her. By the end, she had muscled her way up to main character status. I actually had to go back and do research to figure out which Greek god I could make her a descendant of. I hadn't planned that far ahead for her.
I’m not sure why certain characters take on lives of their
own, but Kara certainly did. She took my book in a direction I never thought it would go, making her own decisions and invading scenes that weren't meant for her. In the end, I'm so grateful I have these crazy voices running through my head at night...without Kara, my book would be drastically different, and my readers wouldn't have one of their favorite characters.
Turns out, a little crazy does a writer good :)