I've always liked to capture the kind of natural expressions that people display when they are not thinking about being photographed, even if they are quite aware at some level that a camera is being used in the area. On the other hand, there are some people who are just "naturals" in front of a camera. We've read about actors and actresses who are "loved by the camera", who apparently always look great with minimal effort on the part of the director and cinematographer. Then of course there is the art of acting itself, which entails, among other things, looking the part. However, in my little world I interact mostly with "ordinary people", and yet it strikes me that some of them have a great natural ability to be themselves and let all their personality show, at the same time that they are quite aware that they are being captured in photographs. The amazing thing is that sometimes you see this in two and three year old children. I suspect it may be a little more common than it was when I was a child, simply because kids grow up surrounded by cameras, and because they are now digital, parents or other photographers can say "do you want to see yourself?" and immediately show the image to the child. None of that takes away any of my wonder when I get the chance to work with a young customer (well, of course the parents are the customers) who is so at ease in front of the lens.
Even so, I'd say it's hard to deny that catching the very young ones when they get caught up in play, and are not doing any "acting" for the camera, is one of the best ways to turn simple moments into keepsakes.
And then sometimes you get surprised and rewarded by a smile and eye contact!