There are many ways to write fiction. You can plan your book, outline it and pave every step of your way to the conclusion -- before writing a thing. I hear some writers do this. Others have a vague idea, sit down and start writing. For writers that ride the seat of their pants, it comes out how it comes out. They're winging it all the way.
My approach is somewhere between the two. I do plan the book consciously. I know how it will begin and how it will end, and I have a mushy understanding of how it will get from here to there. I know my characters, who they are and what it feels like to be them -- though this knowledge expands throughout the process of writing.
When I write a scene, I've set it up mentally before I begin to write it. I know which characters will be in it, what needs to occur in the scene, and I have a notion or two about how to move the action in the required direction. The rest is ad lib.
But that is not the half of it. While writing, I find that great ideas come from nowhere. I can't be any more specific about their source. They're certainly nothing I plan beforehand. They just sort of float into view as I'm writing.
(I also have the strange notion that they emerge into the upper left quadrant of my field of vision. That's where I "see" the idea come into being. This may sound odd but that's how it seems -- it literally comes "out of left field.")
Where do these thoughts come from? They provide the nuts-and-bolts ideas that make the book work. A lot of my best plot turns arrived in this strange manner, and connections between disparate plot areas also occur to me as I write. It's like there's another level of communication going on between various areas of the brain, and this communication only occurs in the act of writing. That's kinda cool.
I guess these ideas bubble up from a well of creativity inside us, though of course this is a uselessly vague statement. What well of creativity? Where is it and how does it arise? In the end, it's a complete mystery to me how any of this happens.
At some point an idea comes into existence, like Athena bursting from the forehead of Zeus. Why do these thoughts emerge from the mists, and at whose behest? These are interesting questions to which I have no answers.
Regular readers know that I just rack it up to Phil, my muse guy. Thanks, Phil! But whether it's Phil or a part of my brain I don't even know is there, the moral is the same: writing is a partnership. It's not just our conscious mind that writes fiction. It's something more.