Behind the scenes of "Buffy"'s musical episode -- The Slayer sings and creator Joss Whedon flirts with tunesmithing

A funny thing happened on the way to this week's episode of "Buffy the Vampire Slayer." Although TV's cult hit has always been a genre-busting anomaly -- combining elements of horror, gothic romance, soap opera, satire, and slapstick -- you could be fairly certain the characters wouldn't break into song.

But now Buffy's going Broadway, and it's all Stephen Sondheim's doing, really. The legendary lyricist-composer ("West Side Story," "A Little Night Music") is a god to "Buffy" creator Joss Whedon. "I know the words to every one of his songs," admits the self-described musical geek. "Well, except 'Passion,' which I've excised from my brain. It was just wrong."

Whedon has been dreaming of staging an all-singing, all-dancing "Buffy" since the show's 1997 pilot. "Every season I would ask, Are we going to do the musical episode?" says Anthony Stewart Head (Buffy's Watcher, Giles), who displayed tasty vocal chops in a 2000 sequence. "Joss would say he wasn't ready. It had to be organic." Whedon's hesitation was twofold: He wanted the episode to be "a normal hour of 'Buffy"' that forwarded existing plot points, not an out-of-sequence stand-alone. Plus, he needed to find the time to write the words and music himself -- a virtually impossible task until this season, when he handed off day-to-day show-running to exec producer Marti Noxon.

As it was, the musical homage (airing Nov. 6) took a grueling six months to make: three months banging out the score on a piano Whedon learned to play just a few years ago (despite possessing only a tenuous grip on music composition, he had no collaborators) and three months of voice and dance lessons for the actors, not to mention all the lip-synching, choreographing, shooting, and editing. "It was a nightmare," says an exhausted Whedon. "The happiest nightmare I ever had."

The show's star shares his beleaguered joy. "I'm not a singer, and I hated every moment of it," says Sarah Michelle Gellar. "It took something like 19 hours of singing and 17 hours of dancing in between shooting four other episodes." Gellar's initial impulse was to use a voice double, but she nixed that after hearing her songs. "I basically started to cry and said, 'You mean someone else is going to do my big emotional turning point for the season?"'

And boy, are there turning points. As Whedon points out, "songs in musicals allow characters to sing what they can't say. And in the case of our characters, the things they really shouldn't say." The catalyst for all the soul-baring is a demon named Sweet, "who thrives on chaos -- and good musical numbers," says Whedon. "He puts a spell on Sunnydale because he knows song and dance will eventually destroy the town -- that much heart opening is too much for people."

The resulting 35 minutes of music (11 full songs, plus fragments and an overture) and 13 minutes of dialogue -- adding up to a longer-than-normal episode -- is classic "Buffy," a seamless blend of hilarity, high drama, and self-mockery. (Whedon found it too painful to cut his musical baby down to regular episode length and UPN offered to perform the surgery, but net execs liked it so much that they're letting it run almost eight minutes over for its initial airing.) "Buffy's first number, 'Going Through the Motions,' is a straight-up Disney production number -- wicked Disney," says Whedon. But mostly "there are a lot of ballads, because the characters are going through emotions -- and because I, you know, kind of go to a sad place when I write." Exceptions include a harder-rocking tune for platinum-haired bloodsucker Spike and an "old school" number for eternally squabbling couple Xander and Anya -- a '30s-style song that, as Anya enviously points out, is "retro pastiche that's never going to be a breakaway pop hit," unlike "Under Your Spell," the (rather racy) love song Tara croons to fellow witch Willow.

"Most of my stuff has a '70s influence -- Neil Young, the Dead, Paul Williams' incredibly underrated 'Phantom of the Paradise' soundtrack," says Whedon, who will release a soundtrack through his Mutant Enemy production company. "Somebody said the songs sounded like Sondheim and early Elton John. Hey, I can live with that."