There are a lot of things to manage when photographing someone. For me, expression matters most. We all grow up reading our parent's faces, so we detect anger, hostility, friendship, humor, warmth,...and the list goes on. We all have this skill and in photography expression trumps all for me. Certainly body language is important, but expression screams at me. The body positioning simply reinforces what is being said by the face. For example, here is a defensive expression, hostile, "don't tread on me" kind of look. Her knees up reinforce this "look".
Here is a "wanting" or "sad" expression, not hostile at all. In part the difference in the two is the angle of the head. Both are looking up, the first image is a head down, the second a head up. Granted these are extremes but illustrate the point.
Expression conveys beauty as in this image. A tilt of the head, pursing of the lips all give shape to the face, making the features attractive. In effect creating the "high cheek bones" for the camera.
In this image there is a similar tilt of the head, different use of the lips, a more playful but serious pose because the mouth is straight. And of course, a different camera angle which defines the ultimate look.
Every face is different and I like to explore the expressions, for story telling. If you get the wrong expression, nothing else works. It's the foundation of the image. If you are shooting outside of the studio with less time such as event or street photography, you are all alone in finding the right expression. If you don't have practice with your observation skills, it is a disadvantage. It is part of the training for the "mind's eye" that is so important in photography. It doesn't matter if it is landscape photography or studio portraits, vision is king.