When someone knows that there picture is being taken, they instantly get stiff and have a forced smile on their face. Forced smiles do not make for a great picture. Especially when your getting paid to take their picture.
Recently I did a location family portrait and I have to say that one of my favorite pictures occurred when they thought I was doing something else but my camera was still on them. As a photographer you have to be ready to capture the moment because you nevr know when it will appear.