Because he monkey

Non-writers misunderstand what we are talking about when we say the characters have a mind of their own, or the characters took over.

First off, they don't understand that it isn't a bad thing. When a story is "alive," you write as if you were merely recording what has actually happened. When a story is "dead," you are putting words down on a blank page.

In practice, you slide back and forth between these, usually feeling a little of one, a little of the other. Thing of it is? The novel I'm working on right now feels like words on paper. When I write, I am creating. The only thing that is keeping any confidence alive is that when I real the two previous novels in the series, I experience them. The words do that thing good words in a work of fiction do; they get out of the way, forming a mere proscenium to frame the world within.

First Person was a difficult choice and I feel that I am starting to push up against a fatal flaw in the series; and that is that my protagonist is always aware of playing a role. Penny can never relax into simply "being" Athena Fox, the way a third-person narrative would present her. She is always inside, working the wires, cruelly aware of Imposter Syndrome.

Perhaps I can make France different. Of course, this particular book is a book of masks and mirrors, of honne and tatemae and gender roles and social role-play. Penny gets to try out lots of faces here; the "grinning motormouth in a Chelsea FC hoodie" she describes herself at one point, the worldly Athena Fox she thought she was creating, the super-spy narrative Ichiro is role-playing (and is trying to drag her into)...

She's a hard enough character to inhabit anyhow (another of the joys of First Person). For this book, what makes it even harder to understand what Penny is going through is that she doesn't understand what she is going through.

And maybe that's why I'm only now getting my throughput up to a thousand words a day.

But back to character. Why does this work? Why do we have an innate sense of what "makes sense" for a character and have trouble forcing a plot-driven choice on them? Why is it things like Character Interviews work, where you pretend to talk to your character and they reveal personal details you aren't aware of creating? How is it that stuff shows up in dialog that you didn't predict?

Because you monkey. You are a primate, with millions of years of experience in constructing a Model of the Mind in order to understand the other members of your troop. And that's why having characters that go in directions your outline didn't predict isn't a bug...it is a feature.