On a crisp November afternoon on the famed New York Street at 20th Century Fox Studios in Los Angeles, old Hollywood comes back to life in an elaborate song-and-dance number that forms the centerpiece of the 100th episode of CBS' hit comedy "How I Met Your Mother."

Airing Monday, Jan. 11, "Girls vs. Suits" finds the nattily dressed Barney (Neil Patrick Harris) challenged to adopt a more casual look to please a beautiful bartender (Stacy Keibler). One result of the tension is a 2 1/2-minute production number to the song "Nothing Suits Me Like a Suit," written by series creators Carter Bays and Craig Thomas.

With music arranged and recorded with the help of composer John Swihart, and choreography by Zach Woodlee ("Glee"), regular director Pam Fryman races the dying of the light to capture the biggest dance segments, which begin shooting with Harris and about 70 diverse dancers (plus a dachshund) dressed in suits.

At one point, Harris leaps aboard a yellow cab and executes a full 360-degree spin -- on the painted metal, in dress shoes -- while lip-syncing into the camera.

"Doing the 360," says Harris in a phone interview a few days later, "that was my idea. He didn't choreograph that. I just started spinning. It seemed like the right thing to do. It actually wasn't very scary, because it all seemed kind of appropriate in the context of what we were doing, jumping up there with all those people on board.

"My adrenaline was kicked to a heightened level, so I wasn't nervous. I didn't really give it a second thought, honestly. I've taken circus-y kind of classes, so if I ever were to fall off the taxicab, I think I would land well."

Of course, there were dozens of dancers around, so perhaps they could have caught Harris.

"You would hope they would do that," he says, "but you never know. They could just watch me fall and then tweet about it."

Among the show's regular cast is former "Buffy the Vampire Slayer" star Alyson Hannigan, who also starred in that show's full musical episode, "Once More, With Feeling," which aired in November 2001.

It's ironic that Hannigan has now been in two TV musicals, considering, as she says on the set after the end of filming, "I just thought, 'Please don't make me sing.' "

"That's always her first thought," says executive producer Thomas, who has been at Fryman's shoulder all afternoon.

"That's what I say when I wake up," Hannigan says, "I just don't want to ruin it."

"Buffy" creator Joss Whedon has also been on the 20th Century Fox lot, working on his Fox series "Dollhouse," so it wasn't a long walk to the New York Street.

"There's no bigger 'Buffy' fan in the world," Thomas says, "as Aly will tell you. I constantly embarrass myself around her and Joss Whedon, who stopped by here yesterday for a second. I totally dorked out on him and scared him away."

But, says Harris, "We were in a very strange part of the filming. We were doing little, tiny, chunky pieces. I wish he would have come and seen the 70 dancers. That's much more enthralling."

As the afternoon rolls toward the filming of the big finale, gourmet cookies arrive, and the other members of the "How I Met Your Mother" cast -- Hannigan, Jason Segel, Josh Radnor and Cobie Smulders -- show up to do their part.

"I'm not sore," Radnor says the next day. "That says what an accomplished dancer I am, if nothing else." As for the song, stage veteran Harris is impressed.

"What you think would be parody and laughable," he says, "is actually quite a good song. It's a very catchy tune. They wanted to do sort of an MGM (musical) but also like 'Hair: The Musical,' that exuberant, high, waily, kind of raspy ode to joy."

Thomas admits to not stopping grinning all afternoon.

"Suddenly," he says, "we're on an MGM musical on 'How I Met Your Mother.' It's ridiculous. It feels so epic. I can't believe we got to do it. I can't believe no one came in and stopped us at some point: 'This is insane. What are you doing.