If someone is preoccupied at the time of being photographed, either physically or mentally, they are photographed as ‘alive’, as authentically living.
To pose or to be aware of the camera – to the point where it makes you aware of yourself and hops your mind onto the circular track of “what do I look like?” – is to be dead when alive.
Photography’s major disadvantage is its ability to faithfully record inauthenticity.
If someone is photographed with their gaze directed away from the lens axis, there is a good chance that an authentic portrait will result. It’s not guaranteed however.
For a portrait to be made with the sitter gazing straight down the lens, staring at their fate – to be trapped in silver – will almost never result in an authentic representation of that person as they are or were in their time here. The only time an authentic portrait can result from staring at the lens is when the photographer is skilled enough and sensitive enough to strike while the sitter’s mind is genuinely elsewhere.
This is a great example of this last point from 1960 by French photographer Jean-Louis Swiners:
It’s a tricky and delicate job and the result is always emotionally gripping.