"Once More, with Feeling" explores changes in the relationships of the main characters, using the plot device that a demon—credited as "Sweet" but unnamed in the episode—compels the people of Sunnydale to break into song at random moments to express hidden truths.

The musical format allowed characters to stay true to their natures while they struggled to overcome deceit and miscommunication, fitting with the sixth season's themes of growing up and facing difficulties in adulthood.

Throughout the series Buffy Summers (Sarah Michelle Gellar), in her role as the Vampire Slayer, is assisted by her close friends, who refer to themselves as the "Scooby Gang".

[2] Buffy died at the end of the fifth season ("The Gift"), sacrificing herself in place of her younger sister Dawn (Michelle Trachtenberg) in order to save the world.

Left to take care of Dawn after the death of their mother Joyce Summers (Kristine Sutherland) in the fifth season ("The Body"), Buffy has come to depend more heavily on Giles.

In the fourth season The Initiative, a secret military organization whose mission is to evaluate and eliminate demonic beings, rendered Spike harmless by implanting a microchip in his head that causes him intense pain when he attacks humans.

Buffy learns that the whole town is affected when she looks outside the shop to see a large group (led by series writer and producer David Fury) singing and dancing about how a dry-cleaning service got their stains out ("The Mustard").

Upon divulging this truth, Buffy gives up on singing and dances so frenetically that she begins to smoke — on the verge of combusting as Sweet's other victims have been shown to do — until Spike stops her, telling her that the only way to go forward is to just keep living her life.

Michelle Trachtenberg (Dawn), who is trained in ballet, requested a dance sequence in lieu of a significant singing part,[9] and Alyson Hannigan (Willow), according to Whedon, begged him not to give her many lines.

[6] UPN, the television network that aired Buffy's last two seasons, promoted the episode by displaying Gellar's face on billboards with music notes over her eyes, and held a special premiere event.

With a characteristic dry demeanor, Giles explains that he overheard the information about Sunnydale residents spontaneously combusting as he was eavesdropping upon the police taking "witness arias".

[7] According to Buffy essayist Richard Albright, the lack of polish among cast members' singing voices added to the authenticity of their breaking out into song for the first time in the series.

[18] Buffy essayist Marguerite Krause asserts that the monsters and demons faced by the Scoobies are thin symbolism for the show's true focus: relationships and how to maintain or ruin them.

[22] Whedon commented that he was "obsessive about progressing a plot in a song, about saying things we haven't said", comparing the musical theater format to the fourth-season episode "Hush", in which characters begin communicating when they stop talking.

[6] According to Buffy essayist Zoe-Jane Playdon, earlier episodes' "false saccharine behaviour" impedes the characters so crucially that it summons a demon to force them to be honest.

[23] The consequences in the episode of concealing truth, spontaneous combustion, is an allusion to Bleak House by Charles Dickens—of whom Whedon is a fan—where characters also face immolation for being deceitful.

Buffy continues her charade in the chorus number "If We're Together", beginning the song by persuading others to join in one by one, as if each is convinced that she is still invested and in charge, and their strength as a group is infallible.

[26] Anya also avoids the truth by burying herself in wedding plans without thinking critically about what being married will entail; instead she considers Xander an accessory to her desired lifestyle.

[30] In The Psychology of Joss Whedon, Mikhail Lubyansky writes that, although Buffy's first step toward re-engaging with her life is telling the Scoobies the truth in the song "Something to Sing About", she does not find meaning again until the end of the season.

The song ends with chord influences from Stephen Schwartz's Pippin and a visual tribute to Disney: as Buffy stakes a vampire, it turns to dust that swirls around her face.

[6][33] Whedon chose the most complicated scene, with the most dancers and choreography in the classic style of musical theater, to accompany an 18-second song ("The Mustard") "to get it out of the way" for more personal numbers later in the episode.

Xander and Anya's duet—the most fun to shoot but difficult to write, according to Whedon[6]—is inspired by Fred Astaire–Ginger Rogers comedies as evidenced by the silken pajama costumes and art deco apartment setting.

[42] Benson remarked that Tara's story arc is significant within the episode, starting out with ecstasy but soon recognizing the illusory circumstances surrounding her bliss and that "life can't be perfect all the time".

Although Salon.com writer Stephanie Zacharek states "(t)he songs were only half-memorable at best, and the singing ability of the show's regular cast ranged only from the fairly good to the not so great", she also asserts that it works "beautifully", paces itself gracefully, and is "clever and affecting".

Debi Enker in Australia's The Age writes, "Giles (Anthony Stewart Head) and Tara (Amber Benson) are terrific, Xander (Nicholas Brendon) and Dawn (Michelle Trachtenberg) struggle valiantly, and Willow (Alyson Hannigan) barely sings a note".

[50] Tony Johnston in The Sunday Herald Sun writes that Gellar "struggles on some of her higher notes, but her dance routines are superb, Michelle Trachtenberg's Dawn reveals sensual dance moves way beyond her tender years, and James Marsters' Spike evokes a sort of Billy Idol yell to disguise his lack of vocal proficiency [...] The rest of the cast mix and match like ready-made Broadway troupers."

[53] Scott Feschuk from National Post states that the episode "conveyed the same sense of rampant, runaway genius—the rare fusion of artful storytelling and ardent entertainment, a production capable of moving viewers to tears or to an awestruck rapture".

[54] Writing in the Toronto Star, Vinay Menon calls "Once More, with Feeling" "dazzling" and writes of "Joss Whedon's inimitable genius"; he goes on to say "(f)or a show that already violates conventions and morphs between genres, its allegorical narrative zigging and zagging seamlessly across chatty comedy, drama and over-the-top horror, 'Once More, with Feeling' is a towering achievement [...] The show may be anchored by existential weightiness, it may be painted with broad, supernatural brushstrokes, but in the end, this coming-of-age story, filled with angst and alienation, is more real than any other so-called teen drama [...] So let's add another line of gushing praise: 'Once More, with Feeling' is rhapsodic, original, deeply affecting, and ultimately, transcendental.

John Virant, president and chief executive of Rounder Records, told the Los Angeles Times, "I remember watching the episode when it aired last October, and after it was over, I said to my wife, 'That's the best hour of TV I've ever seen.

At the 2007 Los Angeles Film Festival, a special screening and sing-along was held that featured both Marti Noxon and Joss Whedon giving brief speeches to the audience.