Inside the super-sized Glee reunion on The Flash-Supergirl musical crossover

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Darren Criss is singing his heart out. Grant Gustin and Melissa Benoist are tap dancing. “It’s literally a Glee episode,” Gustin jokes. No, the Fox series hasn’t been revived. The trio are actually reuniting for the first time since their days on the Emmy-nominated hit series for a different kind of musical — and Criss may finally get his revenge.

His Glee character, Blaine Anderson, received a slushy facial from Gustin’s Sebastian Smythe back in 2012, but now the tables have turned, with Criss assuming the mantle of the dastardly DC Comics villain Music Meister in the highly anticipated Flash/Supergirl musical crossover. “It is funny that the last time I worked with Grant, he was the bad guy and he was singing at me, and now I’m the bad guy singing at him,” Criss says. Though it’s a different set, with the quick turnaround and prep, Criss concurs that this all feels familiar. “It’s just Glee,” he says.

In the March 21 episode of The Flash, Barry (Gustin) and Kara (Benoist) find themselves trapped in a movie musical of the Meister’s creation — an hour that has been years in the making. “We’ve always talked about doing it, from the very beginning, more in a joking way,” says executive producer Andrew Kreisberg. Gustin, however, was initially curious. “I didn’t ever think it would be a real thing, so when they told me it was real, I was like, ‘How?!‘”

Here’s how: As the Scarlet Speedster and Girl of Steel experience turmoil in their interpersonal relationships, they’re whammied by the Music Meister, a less mustache-twirling, more manipulative villain who causes victims to break into song. The character was famously voiced by Neil Patrick Harris in the Batman: The Brave and the Bold animated series. “I have made a pretty decent career of only taking over roles as made famous by child stars,” jokes Criss, who also played the titular wizard in the Harry Potter musical.

“I could not be happier that he’s here,” gushes Benoist, whose Glee character Marley Rose was mentored by Blaine. “He’s killing this role. He’s bringing such a fun energy, the way he did on Glee as well.”

Being fans of movie musicals as kids, the power-stripped heroes end up trapped in a golden-age send-up involving rival gangs and their kids falling in love — though expect many of the couples to come with a twist. The only way out? Follow the Meister’s script till the end. It may be a prison to them, but it’s heaven to Benoist. Noting she was born in the wrong era, the actress takes delight in the vintage ambience while sitting on a ’40s-era nightclub set. “Not that I don’t love the [Supergirl] suit, but it just feels good playing the same character in a completely different setting,” she says. “I’m eating this up.”

The Glee alums aren’t the only big names attached to the special episode. In a nod to Kara’s favorite film Wizard of Oz, everyone else in the vocally impressive cast is playing a character within this movie, from Jesse L. Martin, Victor Garber, and John Barrowman as rival mob bosses to Jeremy Jordan’s club pianist and Carlos Valdes’ aspiring artist busboy. Cherry-picking the movie musical genre — West Side Story, Singing in the Rain, and Guys and Dolls among the show’s inspirations — the hour also features some original songs, including “Runnin’ Home to You” from Oscar-winning La La Land duo Benj Pasek and Justin Paul, as well as the bubbly and playful Rachel Bloom co-penned tap duet “Super Friend.” “It’s a goofy song and it’s very consistent with that style of Crazy Ex-Girlfriend,” Gustin says.

“We were approached by Rachel Bloom, who reached out to us and she really wanted to write a song for us,” Kreisberg explains. “And Greg [Berlanti] had a relationship with Pasek and Paul, so we sat down with them — we hadn’t even seen La La Land with them, but Greg had been such huge fans of theirs.”

The episode also includes several covers, among them the Benoist solo “Moon River,” and an ensemble performance of Jackie DeShannon’s “Put a Little Love in Your Heart” that brings the denizens of the ’40s nightclub to their feet as the Meister conducts Jordan, Valdes, and John Barrowman in an epic, upbeat dance number choreographed by Glee’s Zachary Woodlee. “Some things are just star-crossed,” Valdes says of his two worlds — superheroes and musicals — colliding. “Some things are just destined and written in the stars. This felt like one of those instances where it was just something that was going to happen.”

Jordan concurs: “Ever since they announced the cast of our show — plus the casts of Flash and other shows having so many musical theater people in them — it was only a matter of time before they harnessed those talents and made something into it.”

All involved have unquestionable talents when it comes to singing, but Barrowman says the DeShannon piece doesn’t play to his strengths: “I’m exceptionally confident vocally, but they show me the dance steps and I’m not a hip-hopper, no way,” says Barrowman, who practices his moves on set between takes. “I panicked.” Still, Barrowman was determined to be involved once he learned of the crossover. “I hadn’t heard anything and I just basically then said, ‘I think you would be daft or stupid not to have me in this, really,'” he says. “But they planned on it all along.”

On the flip side, Gustin says he was excited about slipping back into his dance shoes. “More than anything else, my true foundation was tap dance,” the actor says. “It’s been cool to get back to it.” Adds Benoist: “It’s funny how things stay in your muscle memory. It was fun to pick it back up again and see your body and be like, ‘Oh, we remember how to do this.'”

With everyone in one room, Barrowman admits, “I’m a bit fan-boying out to be honest,” explaining that he watched his castmates’ former projects Glee and Smash. “What a nerd,” Jordan responds upon hearing the news. “He plays it super cool backstage, so you wouldn’t know he was geeking out.”

Beyond Glee, the hour marks an even bigger reunion as Gustin and Supergirl‘s Chris Wood — who, along with co-star David Harewood, will guest in the crossover, but not sing — went to school together, while Criss counts Valdes, Pasek, and Paul as classmates. “It’s been a very sentimental crossover on so many levels,” Criss says. “Every day, every scene my mind is being blown. I can’t believe I’m here with Melissa Benoist and Carlos Valdes. ‘How do you guys even know each other? Oh, that’s right, you’re both superheroes!'”

Fun fact: Another classmate of theirs, Andy Mientus, also previously played a villain on The Flash, with Criss joking it’s about damn time he appeared in the Berlanti-verse. “I was wondering why everybody else I knew had been on the show but me,” Criss says with a laugh. In truth, Kreisberg says Criss was destined to play this role. “Darren so brings him to life,” the EP says. “I’m not sure who we would have gotten to do it if Darren had said no, but it turned out to be one of those things where we wanted Darren and Darren actually wanted the part. It was very kismet.”

Should the crossover prove successful, Kreisberg even has hopes for a sequel, and Criss would be elated to return to the wild, wild world of singing superheroes. “I would have liked to have written some songs,” quips the actor and musician, who landed the role too late in the process to do so. “I want to come back, I’m not done — then I’ll have my true payback!” Watch out, Flash!

The musical crossover will kick off at the end of Supergirl‘s March 20 hour, with the majority of the action taking place during The Flash‘s March 21 episode, both airing at 8 p.m. ET on The CW.