...Pletnev is a nice example of a pianist who barely moves a hair when he performs...
Anyway up until recently, I move a lot, make facial expressions, sniff the keys a lot, and sometimes hit my head on the piano lid

I simply followed the flow of the music - if it's soft I'd crouch, if it's painful I'd cringe, etc. I continue this, despite being told otherwise by my teacher.
However I recently realised that it detracts from what I should be doing with the piano - the sound. Sometimes I got caught with too much emotion that I forgot about the kind of sound I'm making. It is important to note that the sound should be the expressive device - and I'm not necessarily creating expressive sounds, just because I can feel the music so much so that my body follows it.
In fact, now that I move a lot less, my process of thinking changes when I play. While I sit still and up straight, I somehow manage to then think about how I should play every note. No longer am I simply listening to myself play and get carried away, as shown through body movements - I am now focused enough to be able to tell myself what I want to hear next and how to do it.
This is my personal experience. And in restrospect, I'm not saying that you cannot be musical and expressive if you move around. However, I think any pianist will benefit from attempting to sit still and focus merely on the sound that is being made and that should be made. After all, according to Berman, that's a mindset that musicians should have when they practise
...so yes I admire Pletnev for being an extremely focused musician
