Sketches, "napkin doodles," rough drafts, and in-the-moment jams are the raw materials that become, with iteration and expansion, refined realities.
I typically sketch out the pages to all my books (admittedly, poorly). This has big advantages. One, it helps me with the writing process and story flow. Also, I often add things to a page that aren't explicitly called out in the text. Little things to keep it interesting. Two, I want to ensure everything that needs to be in the page is in the page to tell the story I want to tell.
At some point, after so many books and working with the same artist, I'll probably eliminate the initial sketches. But for now, it's a process that works well. (For those that are wondering, the artist isn't bound to my sketches. They can make any changes they think are necessary for the benefit of the page and story).
Napkin doodles. Gotta love 'em!