tZer is right on with everything he said.
Prior to joining the band I'm in now, I didn't move around much. I wasn't a tree either, but I just sort of hung back and grooved.
The band I'm in now is such a popular group (locally anyway), that we are pretty much guaranteed a good crowd. Even on the slow nights, we have a following that goes to every gig, so even if the crowd is generally dead, playing for them is always fun. We know we'll have at least 10-20 people who'll dance and whoop it up with us.
I can't help but get caught up in the fun. I move around more than I ever have...and it's all comes natural. It isn't any preconcieved ideas about what I'm going to do. I just go with the flow. My guitarist is just about THE quitest, shy, introverted person you'd ever meet. Onstage, he just goes absolutely wild...and he doesn't know why. This band just has the philosophy that, "What the hell, we got into this to have fun, so if we're not having fun (regardless of what the crowd is like) why do it at all?"
Every gig we play, we cherish. We're musicians, and we're damned fortunate to be able to do this, have all this fun and get paid too!!! Why wouldn't we be smiling and interacting with the crowd?
If you're not there for the crowd, why are you playing in front of one? And, if you are playing out for the right reasons...and enjoying the hell out of it, most of the time we musicians are entertained by the crowd just as much as we entertain them.
I can't tell you how many women will dance in front of me simply because I acknowledge them and interact with them. I'm a very happily married guy, but they don't know it at the time and are more than willing to "show off" and cut loose. That's why they went out in the first place. Who am I to disappoint them?
If I didn't want to entertain them...then I should stay at home and play for myself, or record something.