It feels like forever since I started plotting Rin's story before Shinigami Eyes ever became a full fleshed out story, and I can tell you that it has changed (a lot). Even though there are some characters that I knew their roles right from the beginning still changed and developed. I knew Matt was always going to be an otaku, Rin was always going to be, well, Rin, and then there's Misa...
Where to start... I don't want to give too much away about Misa, but I can tell you that Misa is Rin's imaginary friend, and also the reason why Rin's sent to Japan in the first place.
I always knew the role Misa was going to play, but the way I wanted to reveal her really changed... At first I tried to be sneaky and do the whole 'unreliable narrator' route with Rin not mentioning that Misa's not even a real girl, so it would lead the reader to think she was just a little sister, but Misa just wouldn't shut up and how could I explain why no one else was talking to her?
So, I let Misa dictate how she wanted to be treated, and invisible friend it is... I find out that if you go in with a very strict plan about where you want your characters to be at the end of the story, or what you want them to do, then they have no real time to grow... My motto is let the characters be free to disrupt the story, just hopefully not do something as silly as get thrown into a padded cell (that's another character in another story, still working the kinks out of that one... yeah, where can you go when your character wants to be declared cray cray???)
What's the craziest thing your characters have done that wasn't what you had originally planned out?