VIDEO: Go Behind-the-Scenes of FLASH/SUPERGIRL Musical Episode

ET goes behind-the-scenes of THE FLASH/SUPERGIRL musical crossover episode in the exclusive video! The episode will feature Broadway's Jeremy Jordan, Jesse L. Martin and Darren Criss in a guest starring role as The Music Meister. In addition, the show will feature the original song "Runnin' Home to You," penned by the Oscar-winning songwriting team of Benj Pasek and Justin Paul (LA LA LAND, DEAR EVAN HANSEN) and the original song "Super Friends" penned by CRAZY EX-GIRLFRIEND creator and star Rachel Bloom and performed by Gustin and Melissa Benoist.

"We're not faking it," Gustin tells ET. "This is our background. All of us are trained in musical theater." Watch the video!

The Flash/Supergirl musical episode titled "Duet," airs on Tuesday, March 21st and is described as follows: "Barry (Grant Gustin) and team are surprised when Mon-El (guest star Chris Wood) and Hank Henshaw (guest star David Harewood) arrive on their Earth carrying a comatose SUPERGIRL (guest star Melissa Benoist) who was whammied by the Music Meister (guest star Darren Criss). Unable to wake her up, they turn to Team Flash to save her. However, the Music Meister surprises THE FLASH and puts him in a similar coma, one that Team Flash can't cure. Kara and Barry wake up without their powers in an alternate reality where life is like a musical and the only way to escape is by following the script, complete with singing and dancing, to the end."

Video courtesy of ET