Here are details of two preliminary sketches for the painting "Garden of Hope" from Dinotopia: The World Beneath.
On the left is the "charcoal comprehensive." This half-size drawing, which I drew after completing all the thumbnail sketches and photo reference, allowed me the chance to figure out the basic arrangement of light and dark tones, and how the figures overlapped. It was so easy to erase and change things in charcoal that I felt free to add or subtract elements until I was really sure I had the composition I wanted.
On the right is a very quick color sketch over a photocopy of the charcoal comprehensive. It was painted loosely and quickly with big bristle brushes, forgoing all detail.
With those studies behind me, I felt a lot more sure of myself when it came to painting the final image. There are always so many issues to figure out with any painting, that I find it helps to solve them systematically in advance.
Added later -- for John and Frank, here's one of the early color sketches, before getting models and doing the charcoal drawing. This sketch followed many horizontal compositions that just weren't working.