About what? Have you never been to a large stadium concert? The reality in those is a person could be pointing a grenade launcher at the stage from the other end of the stadium and the person on stage wouldn't know it. Probably wouldn't even be aware of the grenade until it was exploding. No doubt a person on stage in a small venue can see the audience but not in large arenas, at probably 50 rows from the stage you would be unlike to know anything the person in the audience was doing, certainly unaware if they were standing, sitting or playing angry birds on a cellphone.