Love this clip. The best part is that it's a surprise for her. Gameshows nearly always let everyone know it's coming, so the recipient is very much "in on the joke," which erodes the humor of it. In this clip, she came from the audience, which is expected to be safe from pies, and even while she was being led into position, she wagged her finger at them as if saying "you're not allowed to pie me" (reminicent of the classic line "you wouldn't dare!"). Then her reaction to being pied was a nice bit of shock plus a funny little slipping stumble to add to her humiliation.
This was the era of CM when they added dancers. This girl was one of them. The dancers were sitting in the audience when they weren't performing on stage.
They selected one dancer every week to participate. Every dancer, male or female, was destroyed when they lost. It appears that the models relished the opportunity to pie them.
So while she was obviously nervous and her reaction was genuine it's not entirely accurate to say that she was caught off-guard.
Update: This is definitely a great clip. I wish the show had more moments like this.
Love you, too
