Throughout its run, The Flash has brought several Glee stars into the Arrowverse in both big and small roles. Grant Gustin, who portrays Barry Allen was mostly associated with the musical dramedy as the villainous Sebastian Smythe. Starting in season 3 until his final appearance in Glee season 5, Gustin played the openly gay Warbler member who had a rivalry with the New Directions. But once he landed the DC role, The Flash has become the actor’s most famous gig as Gustin’s character is one of the front faces of the Arrowverse.
However, Gustin isn’t the only Glee veteran to make it over to The CW in a DC role. Following the premiere of the Barry Allen-lead drama, the casting department has continued to add in multiple actors into The Flash throughout the seasons. This is what eventually led to the infamous musical episode between The Flash and Supergirl that brought several of the musical voices together in the season 3 episode “Duet.” These are the actors that have appeared on The Flash from 2014 and forward.
Grant Gustin (Barry Allen)
Starting out with the first Glee talent in the Arrowverse is Gustin who began his DC TV career in the second season of Arrow that setup The Flash. Prior to getting his powers, Barry had been searching for his mother’s real killer in order to exonerate his father who had been framed for killing his wife. For six seasons, Flash has been one of the primary heroes in the franchise. On Glee, Gustin’s character had a beef with the New Directions primarily with Kurt Hummel as he wanted to get in the way between him and former Warbler lead Blaine Anderson. Sebastian later gets redeemed after he sees how his bullying partially played a part in Dave Karofsky’s suicide attempt.
Victor Garber (Martin Stein)
To Arrowverse fans, Victor Garber is known as Professor Martin Stein who became one half of the superhero Firestorm which started on The Flash. While Steins’s biggest role in the franchise was in the first three seasons of Legends of Tomorrow until his death in Crisis on Earth-X, Garber has a Glee connection which was a small one. The actor guest-starred in the show’s third episode where he played Will Schuester’s father, only credited as “Mr. Schuester.”
Melissa Benoist (Kara Danvers)
The second Arrowverse series to star a former Glee alum as the titular hero was Supergirl as Melissa Benoist got cast as DC’s Girl of Steel. Benoist's first Arrowverse crossover was actually with Gustins’s Flash which brought the Glee stars together. Benoist joined the FOX series during the fourth season as Marley Rose who became one of the new members of New Directions after multiple original singers had graduated from McKinley High. However, Marley was written out along with several rookie New Direction members in the middle of the fifth season.
Darren Criss (Music Meister)
Gustin got a chance to work with his Glee co-star in the musical episode when Darren Criss entered the Arrowverse as the Music Meister. While his introduction began in the Supergirl episode that set-up the crossover, Criss's time as the Music Meister was entirely on The Flash. On Glee, Criss portrayed Blaine, (the rival to Gustin's Sebastian) as he was one of the prominent characters from season two and forward, particularly because of his romance with Kurt.
Max Adler (Jaco Birch)
The last Glee alum to appear on The Flash was Max Adler who played Dave Krafosky on the FOX series while he played a gender-swapped version of the DC villain Hotness in the season 4 episode "Run, Iris, Run." Krafosky was the gay character who almost committed suicide and was bullied by Gustin's Sebastian. Initially, a bully himself, once Krafosky came out of the closet, the character redeemed himself by the time Glee came to an end. Since Adler's appearance in The Flash season 4, no other Glee alums have been cast in big or small parts in the Arrowverse series.