The body speaks volumes in small nuances of positioning. I often will pose people standing as well as sitting. The skeleton provides the framework for the body. A sitting position will naturally bend limbs like arms and legs. A limb that is bent also provides visual interest since it is less static.
If you ask a person to sit, s/he feels like they are “doing something.” Yet this helps them relax. More often than not, a relaxed subject will allow you to “see” into their persona. This makes for a better portrait.
If there is a height disparity in subjects (2 people in the scene/frame) it often alleviates the difference when they are seated.
If you choose to show hands in your portrait (I often do since I feel that hands can help provide storytelling fodder about the subject) make sure you deliberately position them with care. A seated position naturally allows for positioning in a lap rather than dangling arms at the side.
Some people photograph better standing but make sure you try a seated position too. I think you’d be surprised at results. It goes without saying that if you are using studio lighting that you must adjust your lights, but it’s well worth your time to try a seated pose.
Good post and great advice Nano!