A strange thing happens when people watch a cappella groups perform. I call it "a cappella face." Basically, it's that look people sitting in the front row wear, that confused expression that says, "WTF am I looking at?" Even when they like the music, some can't help but make that face. You know that face? You know what I'm talking about?
This dovetails with another thing I like: Videos of a cappella groups performing for celebrities. There's a funny story in Pitch Perfect about a famous Hollywood Christmas party where the Baker's Dozen from Yale perform every year. Marisa Tomei, Hillary Swank, Jim Carrey—they've all been there, they've all seen the BDs. What I really want to know is this: Do the celebrities make that a cappella face? Or do they grove along? Do they mouth the words?
Here's a video of the Harvard Din & Tonics performing for Andy Garcia. Sadly, Garcia's back is to the camera for much of it. But this provides some insight.