I will relate a personal anecdote.
Many years ago I was in a music shop where a friend of mine worked and he was demoing a keyboard for a customer. I noticed that while he played he kept his face and his body dead still, without showing any emotions, like he was thinking of something else, so (when the client walked away) I asked him: "Why is it that you never show emotions while you play? Why dont't you ever close your eyes or move your head or your body?"
His reply: "Because the customer has to think that what I am doing is easy and he will be able to do the same things at home, once he buys the keyboard".