Actually, shut up is used somewhat excessively by the younger generation today. It can be used in a variety of contexts. For example if someone has really hot shoes one might say to them "wow, who did you have to killl to buy those" and the person might respond "no one, they were free" than it would be appropriate to say "shut up!" -in the context, shut up means "oh my god, or I can't believe it"
However, I believe in the context you're referring to you would like people to literally close their mouth... which I will do right now.
p.s. There is always an excuse for Menzel
p.p.s Sorry if that sounded patronizing
"I guess she is one very confident girl who feels if you see it, it shall be.
I hope she sees a room with a piano player and a bunch of scary middle-aged queens sitting behind a table, talking to each other as she auditions!!"
-Marc Shaiman on Kelly Osbourne in Hairspray