Horn, later revealed to be a Southern California voice teacher and musical theater lover, stunned so many with her tremendous chops that she was immediately branded a ringer.
But in a first hand account published by Broadway World, Horn explains exactly how this magical moment came to pass:
Toward the end of the second half of the performance, Kristin wanders on to the pasarel. She held a mic up to a lady in front of me and asked if she knew the song "For Good." Nope. I took the chance, as I was directly behind Kristin, to stand up and wave and say, "I know the song!" This is not like me - to jump up and wave my arms like a crazy person and raise my voice at a celebrity. One of Kristin's backup singers held a mic up to my face so I could answer some questions: "What's you're name?" - "Sarah." "Who's your favorite Broadway star?" I sarcastically hummm as if it's a difficult question to answer. "Do you know the song 'For Good'?" - "Yes. It's one of my favorites." This seemed to peak (sic) her interest. After this, she moved down the line and asked a guy if he knew the song and bantered with him for a few seconds. Afterwards, she said something about going back to pick me because I was a girl. Then, she invited me up on stage.
Tl;dr: Holy crap, harmony.