To add to what Clair said, I think the same thing is true for musical performances. There is nothing worse than watching some one who is visibly uncomfortable sing or play a song. No matter how great they sound, if they look like they want to either throw-up or run screaming for the door the audience is not going to enjoy the performance. Ironically, someone can not be very good but if they are relaxed and just go with it then people tend not to notice that they are not very good.