Dear Bat and Pete
There is, indeed, a lot of the shaman about pianists (and also singers, violinists and conductors). Brendel said that he pulled faces in order to lead the emotions of the audience.
This I can understand, but I honestly feel that when one is playing in earnest there are a lot of emotions that surface. There are a lot of muscles in the face, and most of them convey emotional states. When one is playing, it is almost impossible for these muscles to be inert. However, they are only active, they do not take control.
I like to think of the difference between organists and pianists. Very few organists pull expressive faces. They do show emotions in their face, but truth is, the muscles are active, not under conscious control.
There is only one clear reason for this. While playing, the organist is usually out of sight of the audience. So there is no point in pulling faces and what emotions the organist may express are true ones.
Of course the very fact that the audience does not see the organist makes the organist not see the audience, and therefore he does not react to the public. I have seen many players really attuned to their audiences and of course they expressed emotions: but they were really not pretending. I remember Wieland Kuijken in a marvelous public performance when he quite understood the enthusiasm of his audience and reacted to it.
But Ushida pulls faces even when she is playing to the camera!