Coming out of the show, which was very new at the time, two or three of the singers were outside greeting people and hawking merchandise. An elderly woman in front of me commented to them it was a great show but they had to be lip synching. They asked her favorite performer, ever, and she said Ritchie Valens. They broke into a version of "Donna" for her. As we lingered in the lobby, awaiting my friends, they broke out in "This Magic Moment", I imagine for the same reason.
I've got to hand it to Smokey Robinson. He sure can find them.