Sara Ramirez Sara Ramirez

SPOILER ALERT: This article contains key plot details from Thursday's musical episode of Grey's Anatomy.

The cast of Grey's Anatomy is wearing Seattle Grace scrubs and small, unseen earbuds. There's blood, there's medical jargon and everyone is ... singing Snow Patrol's "Chasing Cars"? While this may sound like a bad Saturday Night Live skit, it's not. Thursday's episode, in which we learn what happened to girlfriends Callie and Arizona after that ominous fade-to-white, is a full-scale musical event, in which cast members sing many of the songs the show has made famous.

"I have been wanting to do it since we made the pilot basically," creator Shonda Rhimes says. "We needed it to make sense for our show. Plus, I had to convince the network, which took me seven years."

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Having a Tony-winning singer-actress in the cast — Sara Ramirez, who won for Spamalot — certainly helped matters. At the close of the March 24 episode, Ramirez's character Callie, who is pregnant, was — spoiler alert! — moments away from flying through the windshield of Arizona's car after removing her seatbelt to retrieve the cell phone her girlfriend threw in the backseat.

Just before the accident, Arizona took a big leap and proposed to Callie. "I think that the proposal is something that is 100 percent heartfelt, but it's just something that she sees as an answer to not feeling legitimate in this new arrangement," says Jessica Capshaw, who will sing KT Tunstall's "Universe & U" with Ramirez. "So, before you can even figure out why it is that she asks Callie to marry her, they're in this horrible situation."

First look: Behind the scenes of the Grey's Anatomy musical episode

Harming a pregnant woman was not Rhimes' prerogative in trying to get ABC to green-light the musical episode. "That story was destined to happen whether or not there was singing in the episode," she says. "It just turned out to be the exact right way for singing to happen." How's that exactly? The musical portion of the episode is seen from a semi-comatose Callie's viewpoint, in which her colleagues both treat her and communicate with her in song. Ramirez points out that Calliope, Callie's full first name, is a mythological Greek figure who was the muse of song. "They make the connection from the get-go, so we understand why it's music that Callie's soul is speaking to her through and why she sees everybody singing."

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(For a better idea of how it will all work, check out this sneak peek of the first minute of the musical episode, which features "Cosy in the Rocket" by Psapp, the song used in the show's original title sequence.)

"When they first said it, all I could think of was song-and-dance, jazz hands and kicking your feet and acting silly," Justin Chambers says. "I mean, at this point in Season 7, we've ventured into every territory you could, so why not do this?" After weeks of vocal lessons from Rachael Lawrence — who has worked with members of the Glee cast — and hours in the recording studio, the cast was ready.

First Listen: The songs of the Grey's Anatomy musical episode

In the scene the Grey's cast is filming, it's all hands on deck because of Callie's extensive injuries. "In fact, it makes me laugh because it seems like there's no other patients in the hospital because everybody is working on Callie," Chambers says between takes.

"Callie's injuries are so catastrophic," Chyler Leigh adds, likening Callie's plight to that of George O'Malley (T.R. Knight) in the Season 5 finale when he was hit by a bus. "It's pretty comparable," she says. "I think the only thing that gives Callie more of an edge is the fact that there's a baby involved too."

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"I don't know how anybody survives that," says Ramirez, who will sing Brandi Carlile's "The Story," among other tunes. "But, I'll tell you something: It's a wake-up call of epic proportions for Callie. What lies in front of her [after the accident] is more than just glass. It's an opportunity to fight for her life and to make sense of why her life, and her life with Arizona, is worth fighting for, if that's the case."

While the action centers on Callie's treatment, "Owen is very much the man of the hour," says Kevin McKidd of his character. The Chief (James Pickens Jr.) has elected him to oversee Callie's case. "I am given the job to manage all the surgeons, who are all desperately trying to be first in line to get their procedure done and to try and save the baby and Callie," says the Scottish actor, who will sing Gomez's "How We Operate."

Meanwhile, it's the relatively green Dr. Lucy Fields (Rachael Taylor) who is tasked with keeping Callie's baby alive. Not all will go according to plan. "Lucy is obviously young to be in the position that she is," Taylor says. "She's probably come from a world of academia and doesn't have a lot of hands-on emergency room experience. That kind of deficit in her training is revealed in this episode."

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"Obviously, what's cool is I get to work with the lovely Kate Walsh as a result of that," Taylor says, as Walsh's character, Addison, who heads Grey's spin-off Private Practice, returns to help save the baby. "She's one of Lucy's heroes," Taylor continues. "She's the person that, when Lucy was training, she wanted to be like her. But obviously, Addison is frustrated that Lucy hasn't taken some of the necessary precautions."

Through the tragedy, Mark, who is the baby's father, and Arizona will find some common ground, following many episodes of discord between the pair as they navigate their unconventional parenting arrangement. "All he's wanted is a kid and now he's going to have one with his best friend," Eric Dane says. "In a matter of seconds, it all changes and very possibly gets taken away from him. So he's kind of freaked-out and a little shocked."

Lexie will be there to support him during this troubling time. "They have a moment where they can just be together and it's OK to cry," Leigh says. "You just know that you're with somebody that you love, somebody that you trust, and somebody that you can break down in front of."

Leigh will be one of the many cast members who will surprise you with her pipes when she sings a stunning version of Anna Nalick's "Breathe (2AM)," which fans will remember from the Emmy-nominated bomb-in-the-OR episode. "I'll never have this chance to be able to do something like this on this show again," Leigh says. "It was an opportunity for me to really be able to explore that side of me that I've always wanted to but just have been too afraid."

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Arizona and Callie's dire straits help to establish a couples theme in the episode, as each romantic pairing at Seattle Grace figures out where they stand, which will be highlighted by an ensemble performance of "Running on Sunshine," by Jesus Jackson.

Because the episode is told from Callie's perspective, "some of us act out of character," Chandra Wilson says. For instance, McKidd reveals that the episode includes a fantasy sequence in which Cristina (Sandra Oh) and Owen "get a big make-out session, which seems kind of incongruous, but it's there for a reason."

The real challenge for the actors came in timing the music and dialogue, which happens simultaneously as some cast members sing, while others, including Ellen Pompeo, Oh and Patrick Dempsey work to save Callie. "It's a game really to make sure that you get all the words in by the end of the music so it all matches," Wilson says.

Executive producer Tony Phelan, who directed the episode, recalls thinking on the first day of filming, "'Oh my God, I made a horrible mistake; this isn't going to work.' Once we started shooting, then I knew it actually could work." He hopes the audience will go through the same experience. "You'll see them start to sing and then you'll just be in it and along for the ride."

Whatever the fan reaction may be, the cast has enjoyed the experience. Taylor says she cried when she read the script. "Watching this happen, I feel like Grey's is doing something really special and it'll be part of the zeitgeist."

The Grey's Anatomy musical airs Thursday at 9/8c on ABC. A full album of the episode's songs — which will also include the cast's renditions of The Fray's "How to Save a Life," Kate Havnevik's "Grace" and "Wait," by Get Set Go — will be available for download on iTunes Thursday morning.