EXCLUSIVE: 'The Flash' & 'Supergirl' Musical Crossover: Behind-the-Scenes of Kara & Barry's Tap Dancing Duet

It's a bird! It's a plane! It's.... a perfectly choreographed superhero musical!

It's a bird! It's a plane! It's…. a perfectly choreographed superhero musical!

The next highly anticipated crossover of The Flash and Supergirl is almost here, and it's guaranteed to be an extraterrestrial extravaganza.

The Flash's Grant Gustin and Supergirl herself, Melissa Benoist will be joined by their fellow former Glee co-star Darren Criss (and many more of your CW superhero favorites!) in an unprecedented musical crossover event – and only ET's cameras were allowed behind the scenes to capture all the perfectly pitched action.

"The worlds are colliding!" Benoist gushed to ET when we were on set with the stars last month in Vancouver.

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So what's in store when the Music Meister (Criss) messes with Kara (Benoist) and Barry's (Gustin) crime-fighting reality?

"Well, we've gone back in time, as you can see," Benoist said, gesturing to her glamorous 1920s-inspired costume. "And we're in this beautiful theater, Barry and Kara are stuck in a dream world and having to figure out shenanigans. They're on a journey. They're both on personal journeys sort of figuring things out together as a team."

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The Fastest Man Alive and the Girl of Steel are teaming up for a duet, and to make matters super exciting, the song has a surprising composer. Fellow CW star and Crazy Ex-Girlfriend executive producer Rachel Bloom lent her musical expertise to the project and wrote Kara and Barry's toe-tappingly fun number, "Superfriends."

"Obviously, I was excited because I love Grant and he's such a good dancer and I knew we were gonna have fun, but then when I found out [our duet] was [written by] Rachel Bloom that was even more exciting," Benoist gushed.

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Gustin and Benoist confessed that they were both having some major de déjà-Glee vibes during the rehearsals for this super catchy duet.

"It just feels so familiar already and we have Zach Woodley, who choreographed on Glee, back again, and I guess during a dance rehearsal, he was using our characters' names from Glee," Benoist said with a laugh. "He was like, 'Now Blaine goes to Marley...' So it's funny."

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"It's kind of similar to the Glee fashion that I have done back in the day," Gustin added of the quick-paced choreography. "Yesterday, we did an hour-and-a-half tap rehearsal and now we're shooting it today. I didn't know. They were just like, 'Yeah, we're shooting this tomorrow.' I was like, 'What?! We're shooting this tomorrow? OK… great.'"

Luckily, the 27-year-old actor revealed that his typically speedy feet were able to keep up with the on-set musical shuffles. "Actually, tap is how I started as an 8-year-old. It's how I segued into theater and I haven't done it in like 10 years, so we're really having a lot of fun," he explained. "I think it's been a while since she tapped too, Melissa. It's crazy! It's been, for me at least, like riding a bike kind of. Like, my feet are realizing what the sequence is before my brain is."

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As for the actual singing, we were pleasantly surprised to see that both Benoist and Gustin were belting out "Superfriends" during filming, rather than just lip-syncing the words. "You gotta have some live singing at all times," Benoist said.

"We've got the flip track and we, like, sing on top of it. So it looks like it's live, but yeah, we're singing for real," Gustin continued. "The crossover is just going to be a really good time. There's going to be a lot of people brought together that you wouldn't otherwise see together. And we're not faking it -- this is actually our real background. All of us are trained in musical theater, so it's a real musical episode."

For even more behind-the-scenes videos and articles from our on-set visit, keep it locked to ETonline and follow @LeanneAguilera on Twitter!

The musical crossover extravaganza kicks off on the Monday, March 20 episode of Supergirl and continues on Monday, March 21 at 8 p.m. on The Flash.