When I get really engrossed in writing a novel, I can imagine scenes in my head wherever I am. Usually I don’t, because I tend to whisper lines of dialogue and don’t want to look deranged in public, but often I lie awake late at night with my characters running through my head. Sometimes this is how I plan stories, or nut out possibilities for later down the track.
Naturally, most of what I imagine in my head is nowhere near what happens on the page, but I still have a roadmap of sorts where I want the scene to end up, even if the journey there isn’t exactly what I imagined. Sometimes the scenes I imagine couldn’t possibly fit into the story, so I’m also writing fanfiction for my own novel in my head. Even when a scene doesn’t get used, it’s an interesting character-building exercise, as I get to see how characters will react to different situations.
My planning for NaNoWriMo has been part-logic and part-daydreaming. I’m in the middle of planning my protagonist’s escape from a prison cell and have entertained multiple possibilities through daydreaming. Most of the ideas don’t work out logistically, so I’ve gone back to the drawing board again and again until I found something that worked. I knew the main character had to escape, but, later on, she also has to perform some evil magic on the behalf of an ancient revenant who has possessed her. A breakout wouldn’t be appropriate in this case, which was my original plan, because the main character needed to have access to the ashes of another dead revenant in order to perform the magic.
I’m guessing a lot of other writers use daydreams to come up with plot points in their novels, or to learn more about their characters. If you have time and privacy (so you don’t get carted off to the asylum), it can be an effective strategy.