I just finished another awesome scene in "Black Swan Origins: Nineveh." It came from the point of view of a character whom I had wanted to keep in the background. However, she decided to step forward and star in a chapter where I really needed a fresh perspective. I love it when characters get free will and do that.
When it comes to writing, I've learned to always expect the unexpected. Helmuth Von Moltke the Elder once said that no plan survives contact with the enemy. Likewise, in writing, no story survives contact with paper (or, in this modern age, Microsoft Word). Something sounds great in your head, but then you write it out only to find out that it is ridiculous and needs to be burned immediately, or that it is ridiculous and can only be saved by several revisions. Or you get a stroke of genius that forces you to completely rewrite all of the pages and pages of hard work which you had done in order to make it work. At any rate, if you are inflexible and unable to adapt, you're better off not writing fiction.