Writing fiction is so much fun.

When you start writing, you're in control, but about 1/3 of the way through a longer story, the characters start taking over and leading the way, to the point that if you're not careful, they'll lead you astray from the story you want to write, and they'll start writing their own story--which may or may not be a better story. If you've never experienced this, it's surprising the first time it happens.