Look for interesting light sources  

Darkness was falling when we walked by this wall, but the headlights from oncoming traffic provided the light on the side of the bride's face and the interesting shadow under the groom's hat.  I could have set up an off-camera cash, but I liked the temperature and unpredictability of the light from the headlights.  I photographed this with my 50mm lens at 1/60 of a second.