James Marsters, Sarah Michelle Gellar and Joss Whedon by Kevin Parry/WireImage.com
Joss Whedon couldn't contain himself. So jazzed that his landmark musical episode of Buffy the Vampire Slayer was being screened at the legendary Cinerama Dome theater (site of this year's awesome Paley Fest), he sneaked into the audience to watch one of the numbers before rejoining that old gang of his for group photos before taking the stage for a retrospective Q&A (moderated by yours truly). Like me, I think he would have rather just sat in the theater to drink it in. But there was catching up to do.

I've finally met someone whose passion for musicals outstrips mine (if only because he's genius enough to be able to write one). The reason Joss arrived too late to set up the screening (following my own opening remarks) was because he came straight from wrapping production of his next musical project: Doctor Horrible's Sing-Along Blog, an entirely independent made-for-Internet (but no word yet on where you'll find it) musical-comedy, conceived during the fallow strike period.

Here's what we know: Three acts, or episodes. 30 minutes total. Written with his brothers. Starring, as the title would-be super-villain, the versatile Neil Patrick Harris (whom Joss told he he saw and loved in his and my favorite composer Stephen Sondheim's 2004 Assassins revival) and Whedon repertory favorite Nathan Fillion as the hero. Felicia Day (who played Vi, one of the slayerettes in Buffy's final season) rounds out the main cast. In a nod to Buffy's "Once More With Feeling" musical episode, Joss's old partner in crime Marti Noxon (well remembered for her "parking ticket" aria) has a cameo as well. (Marti introduced the screening, telling a funny story of how she and Joss once sneaked into a sing-along screening of the episode, and how tickled she was when fans threw underwear at her on the screen- referring to a lyric in which she says she isn't wearing any.)

How much do we want to see and to be able to sing along to this Doctor Horrible? Almost as much as the legions of fans who camped out for hours (some overnight, we're told) in hopes of getting into the Cinerama Dome for Thursday's marquee Paley Center event. In a gracious gesture, shortly before it was time to go on stage, Sarah Michelle Gellar corralled the troops to go outside and sign autographs for some of the stalwart Buffy-ites.

Some behind-the-scenes observations of this feel-good night, which went by for me in a giddy blur. Backstage in the waiting area ("It's not very green for a green room," joked Charisma Carpenter, sounding very Cordy-like), the mood was like a high-school reunion (of really gorgeous people), with hugs and laughs and catching up- including congratulations for Michelle Trachtenberg landing her new Gossip Girl gig. Seth Green, a scream on stage, described himself as Sarah's "watcher" for the night, staying close by her side as her official "date" for the event as they mingled. The last to arrive: James Marsters, just off a plane from Mexico City, where he was filming the big-screen Dragonball, but determined not to let the fans down. (Eliza Dushku, soon to star in Joss's Dollhouse, and Alyson Hannigan had earlier bowed out after being previously announced.) Going mostly unnoticed through all of the hubbub: the classic musical episode itself, being shown on a monitor in the upstairs holding area. Emma Caulfield (Anya), as funny off-camera as on, smiled with a cringe when I pointed out that her big number (complete with crazy dancing) with Xander had just started. She said it was like looking at an old yearbook, though she (like everyone) is proud of the show. A fan gave her a little stuffed bunny (Anya's biggest phobia) before she went out on stage. I'm not sure what happened to it.

For the actual what-got-said, check out Michael Ausiello's report. For me, this was one of those brushes with greatness I know I'll never forget. I'll be humming the highlights for years.