My first group / public performance

.... at a friend's birthday party, cabaret style.  Fifty guests sitting at tables. It was intended as a very fun evening with many kind of entertainers including a belly dancer and non-stripping strippers, so a few, or a lot of mistakes were OK.

The birthday boy played piano, and I theremin for Ketelby's In A Monastery Garden - if you haven't heard of it, this is what it is supposed to sound like. Lessons learned:
* I must be able to continue to play when the other performer stops to re-find his place, swear and turn pages.
* We'd both played this piece a lot, but never rehearsed together, that was a mistake (duh) :-)

Because of the phrasing of the piece, the audience kept thinking ... er , perhaps hoping ... that we were done and applauding after each section.

Later, a vocalist and the birthday boy did Somewhere Over The Rainbow.   Photo here Lessons learned:
* The format was, the keyboard did the melody, vocalist did the melody, then I did the melody. I thought i'd be able to just start the melody on the right note, but I couldn't find the starting pitch - I couldn't hear the theremin's sound over the other performers without the audience hearing it.
* So, I should have done my first note as a glissando from the lowest note up.

Overall: 
* Many people in the audience were totally fascinated with the instrument and several came up to me later to ask about it and try it out.  One guy was sure I had a magnet hidden in my hand.  
* Every time I sat -- I was using a stool -- my pitch and expression fields were completely different - probably owing to a mic stand, people, and other metal things near me. So I should have asked to tune every time.
* My expression field was much much touchier than I expected it to be - again probably due to the amount of metal things around me.
* At one point the master of ceremonies stepped back until his ass was almost on my expression antenna, totally killing my sound.  Now that I could make a good joke about.  "You have a beautiful ass but for now please keep it off my antenna."
* There was a little buzz / hiss noise with the rented board just on the theremin channel. I don't know where it came from.
* My multi effects pedal interacted very unpleasantly with the mixer board / amplified speakers. I don't know what was wrong but there was a quiet buzzing whenever I used it - so I couldn't.
* I would totally do this again. I am looking forward to getting better and performing with others regularly.

Congratulations, sounds like a fun party! If you seriously had only practiced separately and were playing the piece together for the first time in front of an audience then...wow, that's more chutzpah than I could muster! I have a couple of other suggestions that might be helpful:

* I must be able to continue to play when the other performer stops to re-find his place, swear and turn pages.

Depending on where in the music this is, you can sometimes cover for the pianist by holding a long note, or repeating a phrase. Practicing together will also let you know where you can expect that this might happen (maybe even arrange for it to happen if the keyboard player will need the time)

* So, I should have done my first note as a glissando from the lowest note up.

You probably want to get a digital tuner. You can either attach it to pitch preview (if you have it), or get a pedal tuner (most of them will mute when in use. This will help you get confidence with your entrances

Every time I sat -- I was using a stool -- my pitch and expression fields were completely different - probably owing to a mic stand, people, and other metal things near me. So I should have asked to tune every time.

Again, having a digital tuner should help with that. You can quickly go through some "check points" in between songs or when you aren't playing

* At one point the master of ceremonies stepped back until his ass was almost on my expression antenna, totally killing my sound.  Now that I could make a good joke about.  "You have a beautiful ass but for now please keep it off my antenna."

My god I can relate! At the first show I played one of the singers was working the crowd and wandered into my pitch field in the middle of my solo! I still wince just thinking about it. Now I always insist on being at the far right of the group, and if its on a stage I actually like it if my pitch antenna is right at or over the edge of the stage so no-one can get near it.

Daniel, thanks for telling us of your first outing with what is a most finnicky of instruments.

I'm sure it would be possible to publish a book of all the things that go awry at theremin performances and your experiences are not unique so keep on playing and keep on enjoying playing!

Not sure how your set up was but one of the most important things to try and do is to have the loudspeaker behind you so that you are the first person to hear the notes that you are playing. Otherwise there will be a delay albeit slight, which will probably tarnish the performance.

Otherwise, keep on keeping on