What 'The Flash' and 'Supergirl' Can Learn From TV's Unlikely Musical Episodes

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The Flash and Supergirl are about to join the very exclusive club of unlikely TV shows that have hosted musical episodes.

Executive producer Greg Berlanti announced this month that the two DC Comics series will crossover this season in musical episodes featuring stars (and Glee grads) Grant Gustin and Melissa Benoist

"Some of you may know my own personal love of musicals, and actors who have been in them. And we have a number of people who can sing, across all the shows, who'll be making some very exciting appearances in the back half of the year," Berlanti said in making the announcement.

The odds of the episodes being a success are in Berlanti's favor, as both casts feature extremely talented singers. In addition to Gustin and Benoist, The CW's roster of stars on both series includes Broadway vets Jesse L. Martin and Jeremy Jordan as well as pop singer Carlos Valdes. But it takes more than just a tonally talented cast to make a good TV musical episode.

The Hollywood Reporter rounded up some of the best and worst TV musical episodes to see what lessons The Flash and Supergirl can learn in advance of taking on the genre this season.

(This list only includes musical episodes of shows that normally don't break into song and dance, so series like Glee, Smash, Nashville, Empire and Crazy Ex-Girlfriend don't apply.)

Grey's Anatomy season seven episode 18, "Song Beneath the Song"

Reason for singing: After getting severely injured in a car crash, Callie (Tony winner Sara Ramirez) has a near-death experience that causes her to see all her friends burst into song as they attempt to save her life.

Verdict: After 11 seasons, a TV show is bound to have a few missteps here and there, and this musical event from 2011 is on that list. The episode was made up of the cast singing all the iconic pop songs from previous seasons, including Snow Patrol’s “Chasing Cars,” The Fray’s “How to Save a Life” and Anna Nalick's "Breathe (2 a.m.)." However, most members of the cast are not professional singers, nor do they have the ability to carry a tune, aside from some standout performers like Ramirez and Chandra Wilson. TV critics and fans alike bashed the episode as soon as it aired, but the soundtrack to the episode somehow debuted at No. 24 on the Billboard 200. But never forget the sex montage set to "Running on Sunshine" smack dab in the middle of the dark and twisty episode. The wacky premise, the odd script and direction (why did Ramirez have to sit cross-legged on top of her own unconscious body for a song?) and execution of the songs make this a questionable outing to say the least.

7th Heaven season nine episode 15, "Red Socks"

Reason for singing: It was the special Valentine's Day episode and everyone was in love. Other than that … ?

Verdict: If your cast does not have musical talent, it's best to stay away from musical episodes altogether. Stephen Collins may have come from a musical background on Broadway but the rest of the cast was, ahem, anything but heavenly — and it showed.

Oz season five episode 6, "Variety"

Reason for singing: The Oswald State Correctional Facility inmates staged a variety show.

Verdict: HBO's gritty drama about prison inmates surprised everyone when it featured a musical episode with some songs actually set in reality (others were in fantasy sequences). Who would have thought that the series known for its violence and brutal tone could pull off an entertaining and well-executed musical episode?

Psych season seven finale, "Psych the Musical"

Reason for singing: Shawn (James Roday) and Gus (Dulé Hill) work on tracking down the criminally insane playwright Zachary Zander (guest star Anthony Rapp of Rent fame).

Verdict: The two-hour musical episode was written and directed by series creator Steve Franks, and while the fans of the series loved it, it was not as well-received by critics, as the events of the episode took place earlier in the season so there were some errors and retconning that didn't fit into the show's continuity. But the songs definitely captured the heart of the show, so they never felt out of place.

That '70s Show season four episode 24, "That '70s Musical"

Reason for singing: The 100th episode of the hit series was told from Fez's (Wilmer Valderrama) point of view, featuring musical fantasy sequences.

Verdict: The musical episode was a fun installment of the classic comedy, and all the cast members truly got into their roles singing and dancing. However, the episode did not include any original songs: “That ‘70s Musical” featured iconic music from the decade.

Futurama season four finale (former series finale), "The Devil's Hands Are Idle Playthings"

Reason for singing: Fry (Billy West) wrote a rock opera based on Leela’s (Katey Sagal) life.

Verdict: Futurama's first series finale (before Comedy Central brought it back for a fifth season) featured the musical episode “The Devil’s Hands Are Idle Playthings” that followed Fry as he made a deal with Robot Devil (guest star Dan Castellaneta) and swapped hands with him to become a skilled musician, and as a result, win his longtime crush Leela's heart. The Emmy-nominated episode is widely regarded as one of the best of Futurama’s original run.

The Simpsons season eight episode 13, “Simpsoncalifragilisticexpiala-D’oh!-cious”

Reason for singing: Mary Poppins, er, Shary Bobbins stops by to try and spread her joy and optimism to the Simpson family. It doesn't work.

Verdict: The Simpsons has featured many musical moments in its historic run, but the best may have been the musical episode parodying Mary Poppins. Leave it to Homer and his family to turn optimistic Shary Bobbins to the dark side. Written by Al Jean and Mike Reiss, the episode received an Emmy nomination for outstanding music direction.

How I Met Your Mother season five episode 12, "Girls vs. Suits"

Reason for singing: It was the 100th episode, so why not?

Verdict: The Emmy-nominated outing from HIMYM featured a ton of guest stars plus original song "Nothing Suits Me Like a Suit" that allowed star Neil Patrick Harris to show off his musical capabilities. Watching the entire cast sing and dance in the streets of New York, dressed to the nines in their suits along with an ensemble of people in their suits was a real treat as Barney (Harris) reminded everyone that his No. 1 priority was his clothing.

It's Always Sunny In Philadelphia season four finale, "The Nightman Cometh"

Reason for singing: Charlie (Charlie Day) recruits the gang to help bring his rock opera to life so he can propose to the waitress (Mary Elizabeth Ellis).

Verdict: Charlie's musical – based on his song, “The Nightman” – followed Coffee Shop Princess played by Dee (Deandra Reynolds), Dayman played by Dennis (Glenn Howerton), Nightman played by Mac (Rob McElhenney), and the troll played by Frank (Danny DeVito). It was written by the show's stars, and is one of the most iconic and hilarious episodes of the entire series. Lesson learned: No one ever writes a musical just because.

South Park season 12 episode 13, "Elementary School Musical"

Reason for singing: To parody High School Musical.

Verdict: South Park, like The Simpsons, is no stranger to musical moments, but the musical episode centered on skewering the High School Musical fad ranks as one of its best. Stan, Kyle, Cartman, and Kenny try to buy into the High School Musical fad too late after it hit their school. By the time they stop denying the longevity of the fad and give in to the whole song and dance, everyone else was already over it.

Community season three episode 10, "Regional Holiday Music"

Reason for singing: The characters joined Greendale Community College's Glee Club, even after making it clear that they hated the Glee Club in seasons' past.

Verdict: The season's Christmas episode also served as a scathing and clever parody of Glee. Led by SNL’s Taran Killam as the psychotic director of Greendale's Glee Club (cough, Mr. Schue), the episode featured incredible original songs that got to show off each of the cast members' particular set of skills and let Donald Glover's rap alter ego Childish Gambino shine. And if the word "Regionals" is never said again, the world will be a much better place.

Scrubs season six episode 6, "My Musical"

Reason for singing: J.D. (Zach Braff) and Elliot (Sarah Chalke) help treat a patient who hears everything as a musical.

Verdict: This emotional outing of Scrubs is one of the best episodes of the entire series, and the TV Academy seems to have agreed. “My Musical” was nominated for five Emmys, including directing for a comedy, original music and lyrics and music direction. Plus, the episode never strayed from Scrubs' trademark silliness with songs like "Guy Love," "Everything Comes Down to Poo" and "For the Last Time, I'm Dominican."

Buffy the Vampire Slayer season six episode 7, "Once More, With Feeling"

Reason for singing: Someone summoned a demon named Sweet (Hinton Battle) from hell who made everyone sing their innermost thoughts, desires and secrets until they spontaneously combust (or he finds himself a suitable wife to help him rule the underworld).

Verdict: Series creator Joss Whedon wrote and directed the episode as well as all the original songs performed by the cast members. The episode doubled as a major game-changing moment for the Scooby crew when Buffy (Sarah Michelle Gellar) revealed, during her intensely emotional solo song, that they didn't save her from hell like the rest of the gang had assumed … and instead dragged her out of heaven. And she resents them all for it. Finally, her dark moods and lax attitude about everything she used to enjoy in her life made sense. How do you go on living on Earth after you experienced heaven? Plus, the episode ended with Buffy finally kissing Spike (James Marsters) after more than a year of teasing the relationship, and kicked off their epic love story. Fifteen years after its debut, screenings and sing-alongs with the episode are still a staple — including at San Diego Comic-Con. If that isn't a mark of success, we don't know what is.

The bottom line: If The Flash and Supergirl make the premise of why the comic book characters are breaking out into song and dance believable, if they only give songs to actors who can actually sing, have legitimate stakes and reasons for the songs while staying true to each of the shows' core DNA and propel the plot forward, then viewers are in for a great two episode musical crossover. But bringing in Joss Whedon couldn't hurt.

What are your favorite unlikely musical episodes?