^I agree charisma plays a huge part, but it's not 100% of it... It's a mix of charisma, of not being afraid of the public or of the stage, of actually having pleasure of being on the stage.
So once they are there, they feel confortable, and safe, and they give their all to the performance. Most of people are afraid of the judgement of the public, and most of them wouldn't really perform in front of strangers as their daily job.
Even when you check out the performers in pop music, it's hard to find artists who actually seen to be 100% confortable once they are on the stage... You can always find that hint of doubt or fear on their eyes, or they are so reharshed they are pretty much on the automatic mode onde the lights are one. Probably because a lot of them weren't really born to do that, and were mostly pretty people on the right place at the right time.