The rough thumbnailing is easier as you learn to visualize more. I can see a lot in my scribbles- I mentally fill in the color and value and get a lot of the work done in this ugly phase. From there, I get more specific about the main design and slowly add tightness. I tend to start low contrast and add darkness until the end.
The "painterly" style is magic when done right. I'm still learning a lot, mostly by observing. Main points of reference are Michael Turner (aviation/motorsport artist), and Roy Huxley. Of course the center of the target is McQuarrie, but essentially, I want the convenience of digital with the soul and emotion of analog.