While shooting near the Trocadero in Paris, we were setting up a shot with a model in lingerie on a magnificent exterior staircase. As I was composing the scene, I realized I needed a wider lens and went to my assistant standing behind the model to make the lens exchange on the camera. While exchange the lens, the model decide to sit down for a minute to rest and to be less exposed standing around in public in evening lingerie. As I turned around to get back to my original planned position, I spotted her casual 'non-pose' and immediately snapped away a dozen pictures to capture the one illustrated above.
Two lessons learned: What I call the "90 degree rule", sometimes after you get all setup in a compositional position for a photograph, try looking at the same scene from a position 90 degrees to the side of your original position. Often this new angle of perception will give you a creative inspiration beyond your original vision. Second, sometimes the most provocative images are those from unplanned captures ('grabs' as we call them) where the model is not 'posing'.