This past weekend I took some publicity photos for a local organization, and I was dealing with the most difficult photographic subject in existence (besides cats, of course): people.
As soon as you point a camera at someone, they freeze and put on a bland, slightly nervous expression. All the natural lines around their eyes and mouth disappear, their eyes fix on the lens, they square their shoulders... it's a mess. The trick, then, is taking all your photos before the subject can freeze.
My standard technique for capturing candid photos pretending to "get the settings right" on my camera "before I start taking pictures". Since I have a swivel-out, flip-up LCD screen, I can hold the camera at chest level and start shooting surreptitiously while I talk with the person. Then I go through the photos with them and allow them to select the photos they like. (Warn them first that there will be a number of really weird pictures of them in mid-transition from one facial expression to the next.)
This latest photoshoot revealed a new trick: Have an accomplice talk to your subject as you "get your settings right" on your camera. You'll be sure to get plenty of 3/4 view photos, rich in emotion and with very little stiffness. These pre-pose photos are a goldmine.