Jesse Williams, Sandra Oh, Kevin McKidd Jesse Williams, Sandra Oh, Kevin McKidd

If last season's bullet-riddled finale didn't teach you that lives on Grey's Anatomy can change in a heartbeat, then get ready for another harrowing reminder — this time with a fun, whimsical twist.

I was on set for a preview of the special March 31 episode, in which a crisis unfolds involving five-months pregnant Callie Torres (Sara Ramirez) and we're taken on a musical head trip through the doctor's mind. It's a clever device that allows most of the cast, plus guest star Kate Walsh, to break into song.

"We're in Season 7, so a departure episode is always cool," says Chandra Wilson (Bailey). "It's given everybody an opportunity to let off steam," adds Kevin McKidd (Owen).

Rehearsing an OR scene, Kate (Addison), Chandra, Patrick Dempsey (Derek), James Pickens, Jr. (Webber) and Kim Raver (Teddy) are having a difficult time keeping straight faces as Kevin croons "How We Operate," by Gomez: "Calm down; and get straight" he sings. "It's not our eyes, it's how we op-er-ate."

Taking in the absurdity of it all, Patrick cracks, "this is very James Bond," before joining in the fun with air guitar and spanking motions. He even suggests cutting the dialogue altogether and substituting in its place "interpretive dance." Admonishes Kate: "There will be no dancing before we stretch."

What there will be: some "semi pole dancing" moves which Kim says she performs around a bed for Scott Foley's Henry. "Teddy gets a little frisky!"

To be sure, the cast is making the most of a risky gimmick that was previously tested on Buffy the Vampire Slayer, How I Met Your Mother and Scrubs. The muse behind this variety hour is Sara, the cast's one musical pro who collected a 2005 Tony Award for her Broadway run in Monty Python's Spamalot. But even she had initial misgivings. "I'm protective of my character and didn't want anything to seem contrived," she says. "But the context makes sense." While remaining a bit cagey, Sara says Callie experiences "a life-altering event that [allows her] to process why her life with Arizona is worth fighting for."

Before signing off on the idea, ABC executives needed some convincing. So series creator Shonda Rhimes recruited Sara, Kevin and Chandra to perform a cabaret-style presentation of favorite songs heard in Grey's early years. The next step: hiring a vocal coach and luring skeptical cast members like Patrick, Ellen Pompeo (Meredith), Sandra Oh (Cristina) and Justin Chambers (Alex) — none known for their musical stylings — to participate in the background chorus. "Everybody had their own reaction," says Sara. "And understandably so. For some, it can be confronting."

But for Sara, it has been what she calls, "a dream coming true I didn't even know I had. It feels like everything in my life and career has been leading up to this moment."

So, at hour's end will the doctors be singing lullabies or the blues? Sara advises coming prepared for either scenario. "Have a Kleenex nearby," she says. "Just in case it gets to be too much."

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