When "The Flash" and "Supergirl" cross over for their first musical episode, Barry Allen and Kara Danvers will be stuck in the Music Meister's dream world. However, while Barry and Kara are aware of their parts in Music Meister's play, the other supporting characters will not and play their roles accordingly. Now, Entertainment Weekly has revealed just what parts those characters will play.

According to the report, Joe West (Jesse L. Martin), Martin Stein (Victor Garber) and Malcolm Merlyn (John Barrowman) will all be rival mob bosses. Earlier this month, executive producer Aaron Helbing said Joe's daughter Iris will play "a pretty key role in the musical episode," which makes sense seeing as she'll be caught between these three mob bosses.

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Additionally, Win Schott (Jeremy Jordan) will take on the role of a club pianist, while Cisco Ramon (Carlos Valdes) becomes an aspiring artist busboy. The action will take place in a '40s-era nightclub.

“Some things are just star-crossed,” Valdes shared. “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.”

“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,” Jordan added.

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“The Flash” and “Supergirl” musical crossover will start with a lead-in scene on the March 20 episode of “Supergirl,” and continue in full on the March 21 episode of “The Flash.” The episode will include musical numbers from “The Flash” stars Grant Gustin, Jesse L. Martin and Carlos Valdes; “Supergirl” stars Melissa Benoist and Jeremy Jordan; and “Legends of Tomorrow” co-star Victor Garber and “Arrow” regular John Barrowman; along with “Glee” alum Darren Criss as the Music Meister, the episode’s antagonist.

“Supergirl” – “Star-Crossed”

TERI HATCHER (“DESPERATE HOUSEWIVES,” “LOIS & CLARK: THE NEW ADVENTURES OF SUPERMAN”) AND KEVIN SORBO (“HERCULES”) GUEST STAR; DARREN CRISS (“GLEE”) MAKES HIS DEBUT AS THE MUSIC MEISTER – A new villain (guest star Teri Hatcher) comes to National City, putting Supergirl (Melissa Benoist) on high alert. Meanwhile, Winn’s (Jeremy Jordan) girlfriend, Lyra (guest star Tamzin Merchant), gets Winn in trouble with the law. Maggie (Floriana Lima) attempts to help Winn but old loyalties get in the way. The Music Meister (Darren Criss) attacks Supergirl. John Medlen directed the episode written by Katie Rose Rogers & Jess Kardos. Original airdate 3/20/2017.

“The Flash” – “Duet”:

DARREN CRISS (“GLEE”) GUEST STARS AS THE MUSIC MEISTER IN THE FLASH AND SUPERGIRL MUSICAL CROSSOVER – 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. Dermott Downs directed the episode with story by Greg Berlanti & Andrew Kreisberg and teleplay by Aaron Helbing & Todd Helbing. Original airdate 3/21/2017.

Starring Grant Gustin as the Scarlet Speedster, “The Flash” airs Tuesdays at 8 pm ET/PT on The CW. The series also stars Jesse L. Martin, Tom Cavanagh, Carlos Valdes, Candice Patton, Danielle Panabaker, Keiynan Lonsdale and more. Starring Melissa Benoist as the Girl of Steel, “Supergirl” airs Mondays at 8 pm ET/PT on The CW. The series also stars Mehcad Brooks as Jimmy Olsen, Chyler Leigh as Alex Danvers, Jeremy Jordan as Winn Schott, David Harewood as Martian Manhunter and Chris Wood as Mon-El.