Take away the awareness
- Distance. A longer lens lets you work far enough away that the subject never quite settles into being photographed.
- Time. Several answers described simply continuing until people tire of performing — reliable, and slow enough that one contributor measured it in days.
- Step away from the camera. For a formal portrait on a tripod, stand apart from it, keep the conversation going and fire with a remote release. The subject relaxes because the person is not behind the machine.
Give them something to do
The other approach in the thread is to occupy the subject rather than hide from them. One answer suggested asking people to jump, on the reasoning that a physical action is impossible to perform through — the expression that follows is a reaction rather than a pose. The general principle behind it is that people cannot hold a face while genuinely doing something else.
Why the smile appears at all
One answer made the underlying point plainly: people smile in photographs because a photograph is supposed to show that they were happy, and being photographed does not necessarily make anyone happy. So the work is not extracting a better expression but changing the situation — helping them relax, or forget the camera, or actually enjoy the time. The thread was consistent that which technique achieves this varies from person to person.
What to expect
- The most natural pictures in the thread's experience are the ones taken when the subject was not attending to the camera.
- None of the methods is instant, and the ones that work best take the most time.
- Nothing here is about the camera. Every answer is about the situation you put someone in.