ShiroKuro Lovely post. I have been in similar situations this year, playing for our piano group and playing in a fund raising concert to an audience who have paid to attend. Indeed on Saturday 7th Dec I am doing another fund raising concert in which I am covering( at the last minute) for another player's absence.
At the first fund raiser in June I was surprised that I didn't feel much different to times I play for the piano group. Nervousness was still there but I remember concentrating very much on the sheet music which I had selected because it was relatively easy to read and play.
I guess there are two things I try to achieve; firstly to entertain and secondly to share my interpretation of the music with whoever wants to listen. In this concert on the coming Saturday I also want the music to say Merry Christmas.