I've been using Midjourney to fill out my gallery of characters. So far, I've got art for 13 characters finished. I still need write-ups on quite a few, but I think this really helps to fill in my descriptions.
If you're interested in learning more about the campaign all these characters live in, it's chronicled in the Nother Realm podcast!
Most of them have this as a base:
portrait, white background, fantasy art, dishonored style --ar 2:3 --v 5.1
Two of them (Baeth and Father Del) use this as a base:
portrait, white background, medieval, fantasy art, Donato Giancola, craig mullins, parth, masterful strokes --ar 2:3 --v 5.1
Beyond that, I tend to use basic descriptions and generate until something clicks. With two of those, I had to use actors because my prompts didn't return anything I liked. For instance, the nobleman at the bottom, Caundry Aborlein, uses William Powell as a descriptor. I had to Photoshop the mustache on him, however. Here's his prompt:
nobleman, curly shoulder length brown hair, William Powell, arrogant expression, large nose, fat face, fancy clothes, portrait, white background, fantasy art, dishonored style --v 5.1