Ricky Gervais' Extra-Special Star Power

But some of the greatest moments in both hit series riff from music. In one episode of The Office, Gervais' David Brent whips out a guitar to break up a tedious employee seminar by performing "Freelove Freeway," an original acoustic rocker worthy of Spinal Tap. And perhaps the most darkly brilliant moment in Extras comes with the now-famed song ridiculing Andy Millman, performed by David Bowie, who guest-starred in the second episode of Season Two.

"It was amazing of course, having Bowie on the show," says Gervais. "I've been into Bowie since I was about sixteen. I sent the lyrics and called him up and asked him if he got them, and he said, 'Yeah, yeah ...'" (Gervais switches to a slightly spaced-out, ruminative voice when he does Bowie talking.) "And I said to him, 'We're thinking of the music to be sort of retro, like "Life on Mars" ' — and Bowie said, 'Oh yeah, I'll just knock off a "Life on Mars" for you, shall I?' "

Gervais was giddy but didn't quite know what to expect. "He came to the rehearsal the day before the shoot and said, 'Yeah, I've got this thing ...'" Gervais shakes his head. "It was perfect!" he exclaims. "All the little bits to it. It was amazing, because what he did was, he gave us Bowie!"

"I felt a bit worried about it," Gervais adds, "because we didn't ever go with surreal in the series — and that was nearly surreal, having Andy Millman bumping into Bowie in a club, and having everyone singing along like that. But then I thought, Andy Millman is going to clubs where Bowie hangs out, and Bowie does play piano, and wouldn't it be funny if he just did this? So it wasn't surreal so much as unlikely — and when we saw it, it was just too good."

The Extras finale continues with cameos by big stars such as Clive Owen, who appear as extra-sullied versions of themselves. And Andy Millman reveals a personal streak toward the end that might even be worthy of, well, a Hollywood ending. The special also includes one well-known British pop star — you'll have to watch to find out which, but here's a hint: One of his greatest hits involved a lewd act in a public bathroom. Maybe only someone with Ricky Gervais' talent could enlist such a guy to step in front of the camera for an encore riff about the dark side of the Celebrity Age.

The Extras Christmas Special airs Sunday, December 16th, on HBO.