This question is similar to the planner vs pantser question, but I mean it more in terms of an artist's approach to a single panel or even a single page.
I used to thumbnail, rough, finish ink, color...but as I've gotten old and impatient, (plus my mind is pretty much shot) I've started taking a more intuitive approach. I'll often throw large swaths of color on a page or brown washes and see what emerges and then improvise. The element of surprise keeps me stimulated.
I'll often read over my script for a particular chapter and then start making a bunch of panels, not always in order. Sometimes I'll cut the panels apart, do a closeup, or pan way out and create a new panel. Sometimes I spend a certain amount of time trying to figure out which completed panel should come first and what words should go with it.
Here is how I usually started a panel:
How flexible are you as an artist or do you prefer thinking things out?