If you are about to capture a seated person in a portrait, you have to adhere to a specific strategy. Apart from adjusting the shoulders of the subject and straightening the back, you should pay attention to the thighs.
Assume that the subject is seated on a bench. The subject leans back and seated towards the back of the seat. The issue with this posture is that the entire weight is distributed towards the rear part and the thighs of the subject. Such posture will create unnecessary arches and lines. As a result of this, your subject will appear to be wider than usual.
In order to avoid such mistake and make the subject look really nice, you should get the subject towards the front edge of the seat without thighs touching the seat. Depending on the type of the body language the subject wants to expose, he or she can position the legs; crossing the legs, legs apart or placing one foot crossed over the other.