Speaking to TV Guide, Shonda Rhimes explained that she has always wanted to produce one but admitted that she is nervous.
"We're not going to have an episode where you turn on the television and people are singing for no reason," she said. "It has a reason, there's a story underneath it and there's a point to the whole thing.
"I'm still nervous about it, because you can do something like this and it can come out not the way you intended. I have been working on this very carefully because I want it to still be our show, but also heighten it."
However, Rhimes denied suggestions that a musical episode is a risk, saying: "We're not doing what anybody would call a typical musical episode. The comparisons to other musical episodes are too great, and we wanted it to be different. Nobody is going to be dancing and singing down the halls - that's not our show. Our show is known for its music. I very much wanted to do something that was musical without being a musical."
Rhimes hinted that the episode will include songs that Grey's Anatomy has "made famous" and said: "You will be very surprised to discover that some people sing quite well. That's sort of a little gift to the fans, to discover that people who are not necessarily known for singing have pretty great voices."
Grey's Anatomy continues on December 2 at 9/8c on ABC.