According to TVGuide.com, Grey's Anatomy creator Shonda Rhimes is planning a musical episode for later in the season, which is set to be directed by Kevin McKidd.
Rhimes referenced Buffy the Vampire Slayer's musical episode saying, "[Ours] is sort of the opposite of that in a weird way, they did an all-out, show-stopping, Broadway kind of musical. This is very different."
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Link to TVGUIDE article :
Exclusive: Shonda Rhimes Reveals Grey's Anatomy Musical Details