It would/will take a lifetime to answer these questions for you/for all of us, I can recount what I feel I've learned. What's a pose? Done badly, it's stiff, awkward, and you can see the subject telegraphing something that they don't really feel.
Many of the old classical masters, Rembrandt and Caravaggio et al were simply unequaled in their ability to pose/position their subjects that projected a body language/mood/feeling/reaction that seemed so sincere that it sucked you into the painting. The poses in these paintings are just as timely and up to date as anything you'll see in any book about poses. I think many folks assume that since some of these paintings happen to be hundreds of years old, that the technique in terms of posing is passe', it's not.
Paintings by the classical masters are a great resource for brilliant posing, the body positions that reflect surprise, leaning forward to listen, watching to see what's approaching, a look of disgust, every feeling and nuance imaginable. You've probably checked out Theodore Gericault's 'The Raft of Medusa'.........just the most incredible masterpiece in terms of posing, 15 or 16 people in an 'itty bitty' raft, all crowded together, and posed in such a way as to make every individual in that raft a legitimate story in itself. In the hands of somebody else, this subject matter would've looked ridiculous.
Everybody looks/sits/stands for a reason, even when they're bored and don't know what the hell to do next, and it's all good, if it's something legitimate, when it's a mess is when you end up w/the 'deer caught in the headlights' look, the awkwardness of trying and not succeeding in creating the illusion of whatever you're trying to say.
You look at a picture of somebody trying to smile/show teeth, they're stiff and unconvincing, and if that's the artistic statement you wanted to make, fine, but that's not for me, it's not legit for me under any circumstances, if you were trying to make your subject matter transmit the illusion of sincerity, and failed, that's something else altogether.
There are people divided into two groups, actors/singers/entertainers/dancers/mimes, belong in one group, they're practiced in the art of projecting something that looks/feels sincere, and folks who're not professionals at performance, and I photograph the two groups differently.
Actors practice and explore 'Sense Memory' classes as a part of training in how to control their body language, in 'Sense Memory' class, they might be handed a note(in some exercises, they're not allowed to speak) that the rest of the class watching the exercise is not allowed to see, telling them for instance that they are a 'flower', or a 'tree'............the students practice using inspiration/imagination in being a convincing 'flower' or 'tree', and how close they come to conveying a convincing image of what they're asked to do, hones their skill at controlling and being aware of what their body is doing at all times...........they're essentially practicing the science of 'body language'. They try to project something legitimate without being obvious and corny. The best Mimes are simply brilliant at this.
If I'm not shooting an actor, I never tell a client to smile, or to affect some position, I tell them that the shoot is going to be just like daydreaming, where you catch yourself reliving a moment........... at the beach or some other wonderful moment in your past with some person or place, and you all of a sudden realize you're smiling.
I tell them I want them to forget the lights/wires/stands, and we start talking about them thinking about being at a particular place, or with a particular person, or in a particular mood, and that's how I start in.
One individual I shot, who owned a business which handled managing business affairs of many docters, asked me to shoot him projecting a personality instilled trust in whoever looked at his portrait, I had a few days to come up with an idea as to how to get him to look and feel the feeling he wanted to project.
I had him show up in dark clothes, featureless, so all you noticed was his face, he was intelligent/quick, so I told him to turn away from the camera and close his eyes, and I wanted him to imagine being in the situation I was describing, I told him one of the most important people in his life had just come to him and told him they had terminal cancer, I told him to just imagine this person in the room with him right now, a foot away from him, I told him to think about that, and for him to get into his fighting mode, and that when he was ready, to turn around, and open his eyes, and look into the lens, and imagine that his friend was behind the lens, and just look at him, and let him know with his look, that they were going to fight this disease together, that this person so close to him wasn't alone.
This worked, he was happy with the portrait, I had taken somewhat of a risk, since he might have had someone close to him that really had cancer, and this approach might have backfired, and I'm not really suggesting a 'heavy handed' approach like this as a rule, but my reading of him told me he was quick, and would understand what I was doing, so I did it.
Come up for a legitimate reason for a pose/position, for instance, the subject looks like he/she was looking at something else and just noticed you(you in whatever direction the subject/sitter is looking, something like this, always a reason.
And bottomline, the greatest ability I think a portrait photographer should have, and you've got to learn this, if you can't do it naturally, is the ability to make a complete stranger laugh for at least 30 seconds straight, 5 minutes after you've met them, if you can do this, you're going to be good at portrait photographer, people relaxed and trusting you is as important as anthing you do in taking their picture.
