Okay, well, since you have asked, Alistair, I will offer more thoughts on this subject. This thought about "extra" movements brought some interesting images and ideas for me. For one, I would like to suggest that there is a strong distinction between "feeling the music" vs. feeling one's personal emotion about the music. I think that feeling the music and expressing the music comes from an intense inward focus that requires a constant, deep calm and quiet in order to truly play from and express outwardly (and I think this is one of the great disciplines of not just the art, but of life). Also, I think this is infinitely different than human emotion, and I will even say that I believe it is a mistake to assume that music is about human emotion. If it were about human emotion, then of course it makes sense that one must express this in whatever ways it seems possible to express !
My take on it though has something to do with an analogy that has come to mind in reading over this thread. It is generally accepted that a runner, for example, must streamline his/her motions in order to be as efficient in those motions as possible and essentially, the more streamlined, the more pure the act is and the more it represents what running is. This kind of streamlining allows for the expression of running to become as effortless as possible. I think it would be fair to say that most people would find it quite silly to expect a runner to incorporate extra motions in order to adequately express his/her emotion about the act of running while doing it. Not only would it not make any sense physiologically, it wouldn't make any sense artistically either. Running is about running, not a person's feelings about running. When I watch somebody run, or an animal run, what is beautiful to me is the absolute pure expression of the activity. The kind of mental focus that drops from being sheer intellect to a complete and all-encompassing body focus, well, that is special !
Yes, musical expression and running expression have different cultural expectations attached to them. Even so, my thoughts on how these relate is that the point of the motion should be completely oriented to the actual task at hand. Ultimately, in both cases (music and running), they are to me a sheer expression of life itself, and that is what I would want my personal expression of either one to be.
One could argue that life is about emotion, and I would actually state that it is not. I bring this up to give a better insight into my particular remarks on this subject. I think that people equate a performance that is devoid of emotion as being a performance that is essentially devoid of life. And of course, if a person equates emotion with life, well, then it is true for them. I suggest that there is something deeper taking place in life as well as all of life's expressions, including music.
To take it from a slightly different angle, I think that equating musical life/interpretation with personal emotion is similar to equating solutions to mathematical equations with something like human tears, springing from a death in the family; emotion is just not related, or at least not the ultimate point of such things. Yet, I believe the art forms and principles we are talking about are fundamental in the fabric to all of life.
As far as what I have worked on with somebody like M., whom (to answer your other question, Alistair) I have been working with in both voice and piano (as well as other things), we focus on complete alignment. Inwardly and outwardly. We work on moving away from emotional reaction and distraction, and moving closer to the voice of life, which will guide her artistic expression. In a very real sense, this is a study in intense ear training and is an ongoing practice. We work to align all thoughts and actions with what the ear tells us. We listen for ultimate harmony and act on harmony. We gain composure, balance, confidence, grace, and artistic freedom in harmony and alignment.