Many of the regular cast members on The Flash / Supergirl already had extensive experience performing in Broadway and West End (London) musicals before this musical episode. Jesse L. Martin played Tom Collins in the original cast of RENT, as well as in the 2005 movie version of the musical. Victor Garber (Martin Stein) was in the original Broadway cast of Sweeney Todd, and he played Jesus in the 1973 movie of the musical Godspell; two of his four Tony nominations are for musical performances. John Barrowman (Malcolm Merlyn) has been in Sunset Boulevard and Putting it Together on Broadway and is an even bigger star on London's West End, where he has performed in such musicals as Anything Goes, Phantom of the Opera, Hair, and Miss Saigon. Jeremy Jordan (Winn Schott) was in the original Broadway casts of Rock of Ages, Bonnie and Clyde, and Newsies: The Musical, for which he was nominated for a Tony Award in 2012. Tom Cavanagh (Wells) was in the Broadway musicals Urinetown and Shenandoah. Grant Gustin (Barry Allen / The Flash) appeared in the 2010 Broadway revival of West Side Story. "Legends of Tomorrow" actor Arthur Darvill played the lead role in the musical"Once" on Broadway and in the West End and has written and composed musicals for the Globe Theater. Carlos Valdes (Cisco Ramon) and Darren Criss (The Music Meister) attended the University of Michigan together; they worked together within the theatrical group StarKid Productions.
Grant Gustin (Barry Allen / The Flash), Melissa Benoist (Kara Danvers / Supergirl) and Darren Criss (The Music Meister) all previously starred in Glee (2009).
During their first season, the cast of Supergirl (2015) used to joke about doing a musical episode, as most of them were accomplished singers. This episode brings that idea to fruition, although it actually takes place on The Flash (2014) rather than Supergirl (2015).
When the character of the Music Meister was first introduced (in the animated series "Batman: The Brave and the Bold", he used his ability to hypnotize others to sing in order to control Batman, Black Canary and other heroes and villains. He also was voiced by Neil Patrick Harris while in this episode of The Flash, he was played by Darren Criss. This is not the first time Criss has replaced Harris in a musical role; after Harris left the lead role in the 2014-15 Broadway production of the musical Hedwig and the Angry Inch, Criss was one of the other actors who played Hedwig.
The episode brings together cast members from all four CW shows. John Barrowman plays the long-running antagonist/anti-hero Malcolm Merlyn from "Arrow", and currently appears in "Legends of Tomorrow" alongside Victor Garber, who also guest stars. As such, the episode can be seen as something of a crossover for all of the shows, albeit on a smaller scale than "Invasion!"