Something I keep coming back to again and again is how important it is to balance the pursuit of "feeling" with the technical discipline necessary to create a work of art. Too much of one usually lacks discipline, too much of the other usually lacks feeling and becomes mechanical.
One of the most interesting developments in creating a portrait can occur when you check a measurement. Finding you are "incorrect," you make the change. Suddenly you realize something strange... your incorrect measurement felt better than the correct one! How can that be?! Can't you just simply "copy" nature and get exactly what you are looking for?
One of Mr. Kinstler's teachers John C. Johansen cautioned his students to not simply copy nature, but to "make nature what you feel it should be." He believed your interpretation could feel more like the illusion than simply copying what you see.
While technical skill is very important to the artist, intuitiveness and feeling is as well. Whether it is a landscape, still life, figure, or portrait we should try to recognize the essential characteristics of our subjects, analyzing the mood, motion, and feeling to determine what we want to say.
After all, it is the duty of the artist to interpret what they see. If you really believe Sargent simply chronicled... as he once suggested... then look again!