Instructions to the band - mini comedy genre?

I’ve noticed in a couple of movies a situation in which someone gives instructions to a band right before they launch into a song. It tends to be short, but kind of funny. Two examples:

O Brother, Where Art Thou?
When the guys crash the political fundraiser they find themselves onstage with the band. Delmar realizes he knows just what to do. He turns to them and says: “In the Jailhouse Now fellers… neighborhood of B.”

The band obligingly strikes up, and Delmar sings it wonderfully.

Back to the Future
Marty McFly informs the audience he’s going to play a song that’s an oldie where he comes from (which turns out to be the anachronistic Johnny B. Goode). He then tells the band, “All guys, this is a blues riff in B, watch me for the changes, and try and keep up, OK?”

Two questions:

  1. Can you think of any other examples of this kind of thing from other movies or plays?

  2. Could one reasonably expect a band, even professionals, to capably play a song they knew with no more than those instructions? What if they didn’t know the song, such as in Back to the Future?

If it is indeed a standard blues riff, almost any musician could fake it. That’s one of the great things about the blues, you just pick a key and a tempo, and you’re off. Marty told them the key, and he set the tempo when he began playing. The blues chord progression is a pretty set thing, and any decent live musician would know what it is. (I guess I could forgive a classical cellist if he/she didn’t know what a blues progression was…but probably not.)

With In the Jailhouse Now, I got the impression that the musicians likely knew the song, and just needed to know which key to play it in, which Delmar provided. If they didn’t already know the song, he would have had to provide a bit more detail than he did.