From On Photography, by Susan Sontag
There is something on people's faces when they don't know they are being observed that never appears when they do. If we did not know how Walker Evans took his subway photographs (riding the New York subways for hundreds of hours, standing, with the lens of his camera peering between two buttons of his topcoat), it would be obvious from the pictures themselves that the seated passengers, although photographed close and frontally, didn't know they were being photographed; their expression are private ones, not those they would offer to the camera. -- p. 37