There is a lot of psychology behind the fact that when someone is being observed or when there is a camera nearby, their behavior changes. The majority of stores are well aware of this fact, so they put up a sign that says "Smile, You're on Camera". Whether there is actually a video camera in the store or not, the possibility is presented, thus lessening the likelihood that customers will steal.
In times of yore, when a film photographer would take pictures of a person, he implemented the same psychological concept as mentioned above. At the beginning of the photo-shoot, he would start taking pictures of the person with no film in the camera. The subject was totally unaware. After about five minutes, the hope was that the person's unnatural and fake facial expressions would vanish as they became more comfortable in front of the camera. Once this veil of artificiality melted, the photographer would load film into his camera and start taking pictures that would actually reflect the subject's personality. Although this unique method of portraiture was not too common, it speaks volumes against the common belief that a 'Say Cheese' snapshot is acceptable.