Not being an author, but having met a few, I'm fascinated by an author's inability to precisely direct their characters behaviour and situations.
That's the wrong way to look at it. As an author I've absolutely depended on this because it shows that I have enough of an unconscious understanding of the character that my conscious manipulations of their environment create unexpected responses.
Basically I have to understand the environment enough to be able to predict its outcomes and I have to understand the character enough to have a latent understanding of her responses. You're basically making your conscious and your unconscious minds interact and the outcome is fiction. You can make your characters do whatever you want but it rings hollow - you're basically projecting your life experience with others on this construct in your mind and your subconscious, which has better access to your corpus collosum, sometimes offers up a different result than your cerebral cortex.
I'm sure that terminology is a gross oversimplification. However, the trickiest thing in writing is getting out of your own way and when your characters start to surprise you it's like that moment when your camp fire finally starts burning without you blowing on it.
Shit. Maybe I oughtta write another book.