Mark Waid's "Legion Index" (published by ICG in the late 1980s) credits Jim Shooter with pencil layouts for his issues, which is very unusual for a writer. Over on the mailing list for the Grand Comics Database, a question came up about what that really meant, what role he actually played. Shooter replied back to the list (via a third party), and here is the reply, reprinted here with Shooter's permission.
I did very thorough layouts for every single issue of every book I wrote for DC in my first stint there, 1965-1970. I also provided a cover sketch in color, no less. My drawings weren't professional quality, but they weren't childish scrawls or sketchy scribbles, either. I drew as well as I could. I drew every detail. I designed every new character and every new costume and everything else. It makes me laugh when I see in Wikipedia and other places the penciler of an issue I wrote and laid out credited as "co-creator" of a new character therein, when I was the one who created everything, including the visual. It also is amusing when I see Mort given credit as co-plotter or even co-writer on stories I sent in over the transom that were published as delivered. Oh, well.
There were a few cases where my design for a character or something was, at Mort's behest, changed (usually for the better) by the artist. Mordru comes to mind. But, that was rare.
Most artists followed my layouts pretty faithfully. A few of the guys would simplify or cheat if I called for something too time-consuming to draw. Every artist improved my layouts, some generally, some here and there. Mostly they drew what I called for, as indicated. My drawings were much more than thumbnails. Obviously every artist who worked from my stuff drew better than I did, so the final drawings were way better than mine.
The covers Neal Adams did from my sketches followed the layouts in a general way, but with him, it was more like he saw the intent and drew the picture I imagined but lacked the skill to draw. It was as if he was reading my mind.
In the first letter I received from Mort in response to my first submission, Mort invited me to send another sub and actually suggested that I might become an artist for DC. P.S., he eventually bought that first sub, the Doctor Regulus story, and everything else I sent him.
Back then, DC (or at least Mort) didn't run credits. On the issues I did with Woody, he lettered in a credit for me on the splash. Woody hated writers (and editors and art directors) so I wondered about that. Years later I asked him about it, and he told me it was because I wasn't just a writer -- my layouts qualified me as an artist, and therefore worthy of credit.
Gil Kane said in an interview once that I was the only writer he liked to work with because my layouts saved him so much time. I think he actually light-boxed some of them (and improved them tremendously on the fly -- he was genius enough to do that).
Curt Swan used to send me letters occasionally neatly lettered on huge sheets of vellum replete with many drawings, in which he'd give me drawing tips and advice regarding angles, layouts and storytelling. I learned a lot from him. He was so nice.
DC pays me for layouts as well as script when they reprint that old stuff.
Does that answer the Q?