I've recently been going through old family photographs dating around 1850-1920. This was back when the photo emulsion was very slow, and the subject of the photo had to sit for as long as five minutes to complete the exposure.

These pictures are very obviously not candid, however, they have a very candid look to them. The expressions are natural and piercing, the eyes are clear. My theory is that they had to sit so long in one position that they could not afford to hold a forced expression, and so the result is that the picture shows their face in its natural, relaxed state.

This is in stark contrast to many of the modern professional studio photos I see. Most of the cheaper studios do not bother to try to get photos like this, and are more concerned with moving as many wiggly kids through as fast as they can.

Occasionally, you can accidently get that "perfect snap" just by being there and getting the candid shot. However, if you can manage to get your subjects to calm down, get their mind off the camera, wait a moment for them to relax, perhaps you can do as well as the old pictures did.