First, I decided to place them on a beach. That would give me the opportunity to use a figure to give the scene life. But most important, I wanted to use some dramatic lighting to highlight the rocks. Putting the emphasis on the shadows creeping up on the rock formation gave the scene its focus.
I've seen this kind of lighting late in the day when the light changes very quickly. Setting up and painting such temporary light effects is problematic. Memorizing the look is the best way of recording the light.