Many people, and I am one of them, complain that when they practise at home everything goes fine, but in front of a teacher - or, worse, an examiner - wrong notes appear out of nowhere, the timing slips and it all goes to pot. Sure, some of it may be nerves - but my teacher doesn't make me nervous, and I don't play nearly as well in lessons as I do at home! During my last lesson, I think I found part of the answer (well, an anwer): Body language.
I was playing arpeggios for my teacher - 4 octaves with her sitting beside me. Conscious of the fact that I didn't want to end up in her lap, I was using my hands to play the notes and keeping my body still. Consequence: wrong notes in all the top octaves! I think this kind of awareness of another person's presence must affect all aspects of our playing to some extent. If you are alone, you can fill the room (sort of), but when someone else is present I think most of us shrink down to some extent out of deference

. Perhaps the answer is to "claim the room" every time you are going to play, regardless of who/how many are present. Be a true Performer

and don't be afraid to dominate, accept the room as yours, etc, etc.
So... What d'you think?