I love how pictures turn out when the subject doesn't know you're taking any. People always appear much more natural and real.
Take this kid for example:
Him not knowing I was taking the picture set up the shot so much better than if he was staged. He looks completely melted into being at the beach, enjoying the surf. His natural condition, immersed in the setting. As opposed to a "smile for the camera" moment when all that is lost and he's just another kid at the beach.