Bert, no one is completely useless. There's always a need for good audience members, at the very least.
MMario, As long as it's not mime, it's fine by me. (He said as if his opinions on street mime really matered!:-))
Kim C, Of course historical reenacting can be theater! Now you've got me curious; where do you perform music?
Alice,The Mikado is one of my favorites. I just recently saw a really fun amateur production of it. Ages and abilities ranged all over the map. But, boy, were they having a good time on that stage!
Peter T., your musical shyness is not so strange. I experience it myself. Never when acting. But when I'm performing music up there by myself and only have my voice and guitar/banjo/whatever between me ant the audience, there a palpable wave of nerves. It never happens when I'm playing in a social or jamming situation. But put me in front of an audience and I'm a mess. I've managed to get enough control over it that I now expect it to happen and can deal with it and put it away in the first song or so. I hope you can do the same and get more enjoyment out of sharing your music with others.

All these answers! I don't know why I'm feeling so chatty this morning!
- Sean