Performing music, as with other musicians or singers, requires the factor of control. A player should be in total command of his entire body. It is wrongly thought that body and facial gestures mean showing emotions. Although physical expression has its place, it is not what carries emotional expression. You see, a music performer doing this is probably unaware and therefore not in control, or is aware and is exaggerating and having that be the carrier wave. Let us go deep into this.
Some musicians habitually do such things as ‘jerk’ or bob their heads, move their lips or mouths, work their bodies, raise their eyebrows, obsessively stretch their eyes, etc. Well, imagine if someone did things like that while carrying on a sober conversation with you. The communication will be awkward. Suppose they are using various gestures and motions habitually. In that case, they are mechanically using those to try to get the message across, instead of the purpose and delivery, which is what does it. It is a sort of effort or nervousness. The same policy applies in music, for music is a type of communication.
However, some musicians make a conscious effort to perform physically to impart their emotions. Again, this is an effort to use that as the carrier wave to get the information across. The music itself is the carrier wave. For instance, in parody, the physical movements convey the message. In narrating, the spoken word does it. Imagine if a pianist stated verbally what message they were trying to show, saying, “And now this is where I start being melancholic” or some such thing. It would be awkward. When one is consciously gesturing, they add an undesirable element into their performance.
Now, this does not really mean that one should be dull and unexpressive. Instead, one should be expressive, but there is a proper way to do it. It is done by intensifying the music itself. One does this with the appropriate movement, which is a specific technique. Without getting extra technical, one can practice music with exercise in the climax, characterization of a piece, harmonic progression, contour of the melody line, phrasing, etc.
When practiced perfectly and mastered, the result is harmonious with the music. In actual tests, 80% of listeners, when told to close their eyes and listen to two variations of the same excerpt of a piece, one performed with correct movement (exercise), the other with incorrect movement, could observe the difference.
Any musician of any level, including experts, can hold bad habits during the performance. These are easily fixable for any musician.