You can't perform those physical actions? Were not talking about conjuring a feeling, were talking about emulation the actions that usually accompany them.
Isn't forcing yourself to emulate an expression pretty weird, though? It reminds me of the companies that demand everyone be "enthusiastic" and love their "fun workplace", rather than just allowing people to be quietly good at their job (http://www.kmjn.org/notes/funsultants_and_gamification.html).
Literally, we can't. You may as well be asking us to detach our arm.