Most people don’t have modelling experience (and that’s 90% of the people you will ever photograph), and they feel awkward with their hands.
They don’t know what to do with them. Most leave them dangling on the sides. Posture becomes important too. If someone slouches or positions the head in a certain way that can produce a stiff posture and a bad image.
You don’t want that.
The position of the feet and the legs are also important. I have previously discussed at length about how to pose a bride and a groom. All of those posing techniques are applicable here. It has been written about how to create the letter ‘S’ with a female model’s body.
That posture requires the transfer of weight from the front to the back leg. It brings out the feminine form and creates a much better pose.
For a male model, leaning against a a wall, with one foot resting against it and the other foot on the road/floor, body turned slightly away from the camera and the head turned towards it is a nice pose to start off.
For full body modelling portraits, the best option is not to shoot with too wide a lens.
Wide angle lenses are affected by distortions, especially at the corners and edges. Wide angle lenses will make your model look weird. With a wide angle lens, you cover a lot of the scene. As a result, you have to move in close in order to get a tight shot. That is what creates distortions.
Another reason to avoid a wide zoom lens is they are difficult to work with in small studios.
The best choice is something like a 50-70mm. On the other hand, with tele lenses, you have to stand too far away in order to crop out negative space. That means your subject will be compressed against a background.
Between a wide-angle zoom and a telephoto, however, choose the latter. The look is a lot more natural with a telephoto. And always shoot from a distance and zoom in rather than use the widest focal length and shoot from a close distance.