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Source: Katie Yu, The CW

There seems to come a time in every showrunner’s journey when they stop and ask themselves: Is it time for a musical episode?

Quite often – and sometimes against better judgment – the answer is yes.

“Riverdale” is the latest nonmusical TV series to experiment with the format, as of Wednesday’s episode centred on a school production of “Carrie: The Musical.”

While actors in shows such as “Crazy Ex-Girlfriend” are accustomed to belting out tunes in each episode, these pretend teens only sometimes get to perform a bubble-gum-pop rendition of “Milkshake” while atop a diner roof. (This narrowly beat a weird duet of “Bitter Sweet Symphony” at Veronica’s confirmation, before which she sinned by referring to it as the “song from the ‘Cruel Intentions’ soundtrack.”)

The CW series doesn’t stray far from its normal antics, as the writers embedded the musical into an episode that customarily features the teens clashing over parental drama and threats from a mystery murderer. The “Scrubs” team, in contrast, went all in and turned Sacred Heart Hospital into an extravaganza. Though different in execution, both episodes succeeded; it seems that with at least 20 musical episodes in existence now, there is a precise science to what makes them work.

Here are five necessary components:

1. Featured actors who can actually sing

A two-part episode of “Grey’s Anatomy,” aptly named “Song Beneath the Song,” focuses on a pregnant Callie (Sara Ramirez) in the aftermath of a brutal car crash. As the Seattle Grace doctors treat the injured surgeon, she hallucinates another version of herself performing songs by the likes of Snow Patrol and the Fray. Ramirez has a gorgeous voice – she got her start on Broadway and even won a Tony before joining “Grey’s” – and Chandra Wilson and Chyler Leigh are strong performers as well.

The others? Eh, not so much.

To the show’s credit, Ramirez takes the lead on the majority of the featured songs. “Riverdale” similarly let three members of its main cast – Camila Mendes, Lili Reinhart and Madelaine Petsch – take charge while creating non-singing roles for others. But a question remains: Why wasn’t Ashleigh Murray’s Josie, known in both the show and the comics for being the talented lead singer of the Pussycats, given a larger role?

2. Songs that are woven into the plot

While some series like “Atlanta” regularly embrace the bizarre and could theoretically pull off a nonsensical musical episode – please, Donald Glover – more-traditional shows do not have this luxury. Shonda Rhimes wanted to do a musical episode of “Grey’s” since the medical drama’s start, but she had to wait until, among other things, it made sense for the intricate story arcs. The “Grey’s” team used covers of songs that somewhat related to the episode’s events.

“Scrubs” opted for original numbers, composed in part by “Avenue Q” writers Jeff Marx and Robert Lopez. Debra Fordham, who wrote the episode “My Musical” and most of its lyrics, tailored the songs to the episode. The central conflict involves a patient whose massive brain aneurysm causes her to imagine the Sacred Heart staff singing; her diagnosis is revealed in “When the Truth Comes Out,” a parody of “Do You Hear the People Sing?” from “Les Misérables.” Another gem, “Gonna Miss You, Carla,” refers to the nurse (Judy Reyes) deciding whether to take parental leave from the hospital.

3. An explanation for having a musical episode

A brain aneurysm can fulfil this requirement, as can the established style of a show such as “Community.” The series gained recognition for parodying pop culture, and “Regional Holiday Music” in Season 3 played off creator Dan Harmon’s known distaste for “Glee.” Guest star Taran Killam steps in as the overly earnest coach, and the Christmas-themed episode involves each character reluctantly joining the Greendale Community College glee club.

(Its original members are in the hospital, having suffered nervous breakdowns after they got in trouble for using copyrighted music.) This episode gave us the absolute treat of Abed (Danny Pudi) and Troy (Glover), who was raised Muslim and Jehovah’s Witness, respectively, rapping “Christmas Infiltration.”

Without a reasonable explanation, you end up with the “7th Heaven” musical episode, which is so awful that the A.V. Club dubbed it “one of the worst episodes of TV ever made.”

4. A healthy dose of self-awareness

Musical episodes are weird, to begin with, meaning they lose their magic the more we think about their logistics. But there is a clever way to poke fun at the concept, and “Community” isn’t the only one to do so.

“The Flash” orchestrated a phenomenal episode by pulling from other CW casts – those of “Arrow,” “Legends of Tomorrow” and “Supergirl” – and giving us just enough of an explanation as to why characters randomly break into song. The Music Meister (Darren Criss, making this a mini “Glee” reunion) has trapped the musical-loving Flash (Grant Gustin) and Supergirl (Melissa Benoist) in a singsong dream world, and they must follow the flimsy gangster plot to wake up. This calls for the Flash to utter meta lines like, “Wow, everything is easier in a musical,” while surrounded by Broadway stars.

5. Catchy music

Remember the “Even Stevens” musical episode? Actually, of course, you do. There’s no forgetting “Influenza: The Musical,” in which Ren (Christy Carlson Romano) has a literal fever dream that involves the entire cast singing as she shows up to school unprepared for her science presentation. The songs gave us a catchy way to rail on the sixth period and an easy reminder of our country’s lunar history: “We went to the moon in 1969 – not 1968 but a year laaater.”