The most common casual remark you will about a portrait is that it "really captures the personality" of the subject. This is, taken literally, completely silly. A personality is a big complicated thing, and it's really unlikely that a single image is going to communicate any meaningful subset of it. Add to that: we rarely know the person portrayed, so what on earth does the viewer know about the actual personality of the subject?
As humans we have an enormous amount of mental machinery devoted to
understanding a person's mental state and history from their appearance,
especially their face. We constantly build mental models of what a
person is thinking, based on their expression. A portrait merely needs
to tickle this massive mental machine we already have, to set it in
motion. The machine will fill in a complete person with a history, a
mood, ideas, and personality. Not, of course, the whole thing, but a
skeleton our mind can use to understand the subject, a skeleton which we
will perceive in the moment as a complete person.
What's really going on is that a good portrait evokes a strong sense of some element of personality and, if the photographer is lucky or skilled, of narrative. The viewer gets the sense that they know the subject, a little, they understand a little of the subject's nature and life. We glean this sense from cues in the face and the body language. A smile, a frown, a sad look, each might be genuine or false, each might be perceived as genuine or false. The tilt of the head, the direction of gaze. Our mind processes all this material up. If the cues are consistent and strong, if they are not overwhelmed by a false "say cheese" smile or some other obvious artifice, if they are believable, we'll get that sense of personality. A false smile can be believable, if it appears to conceal something else, appears more than merely a response to a camera. Is the personality we sense genuine, an accident, or is the subject merely a good actor?
How can we convey a sense of narrative, of the subject's history, the life they have lived? Beyond the cues in the face and body: scars,
wrinkles, any evidence of the passage of time writ on the subject's body. Also, clothing, glasses, jewelry, and other props all help support
an idea of narrative. We'll fill in a boxer if we see broken, flattened nose. We'll fill in a widow if we see a ring on the old woman's finger. We'll fill in a hero's story if we see a uniform,
we'll fill in scholar's life if we see tweed and a pipe.
This applies to portraits of people we do not recognize, especially, but also to well-known personalities with whom we have no personal contact. Karsh's famous portrait of Churchill without cigar, Churchill looking cranky, has shaped the way we perceive Churchill. In reality, it reveals nothing of the subject's personality, save that he gets cranky when someone snatches his cigar away from him, which is really nothing. In the viewer's mind, however, we form the impression of Churchill as stormy and implacable, supporting the historical view of him as a the master of the pithy bon mot and winner of WWII.