There are a couple of unexpected stars on the second-season DVD set of HBO's "Extras." Ricky Gervais and Stephen Merchant, sure. But David Bowie and Nigel Williams?

Even Bowie isn't a complete surprise, given that he guest-starred in the season's second episode and sang a memorable ode to Gervais' "pathetic little fat man," Andy Millman. But in a segment of the DVD's making-of documentary, "Extras Backstage," Bowie reveals a well-honed deadpan as he recounts how he agreed to let Gervais and Merchant write the song's lyrics, in exchange for supplying them with jokes for future episodes.

Then there's Williams. Most of the time, he's the editor of "Extras." Some of the time, though, he's apparently Gervais' play toy, as detailed in a featurette called "Taping Nigel: The Gimpening."

The longish and sometimes deeply uncomfortable piece details how Gervais amuses himself by applying tape to Williams' face and head, twisting the poor guy's countenance into goofy and occasionally grotesque masks. Sometimes it's hard to tell whether Gervais and Co. just thought this up for the DVD or whether this is a semi-regular occurrence in the "Extras" offices. If it's the latter, I hope Williams was handsomely compensated for being a guinea pig.

As for the show itself, season two of "Extras" is maybe even more melancholy than the first, even though Andy has achieved a measure of success with a BBC sitcom called "When the Whistle Blows."

To get there, though, Andy has pretty well sold out his vision of a documentary-style show (a la Gervais' previous effort, "The Office") to get on the air with cheap laughs, catchphrases and mugging. And since the show is something of a hit, all the inappropriate and embarrassing things he or his clueless agent (Merchant) do are magnified and potential fodder for the ever-hungry British press. His dim friend Maggie (Ashley Jensen) is no help either.

And, as with the first season, the show is full of celebrity cameos. In addition to Bowie, the likes of "Harry Potter" star Daniel Radcliffe (who's rather fantastic sending up his nice-boy image), Sir Ian McKellen and Orlando Bloom have great fun goofing on themselves. Because they're in on the joke, the cameos never feel gratuitous.

"Extras Backstage" is the backbone of the bonus materials. The hour-long documentary is broken up into six parts across the two discs, with each part roughly corresponding to each of the six episodes. In another featurette the two creators expound on "The Art of Corpsing," the term for the uncontrollable laughter that, judging by outtakes also included, was epidemic on the show's set.

The second season of "Extras" is also most likely its last, as Gervais and Merchant subscribe to the leave-'em-wanting-more philosophy. The series ends on a relatively hopeful note, but somehow you have the feeling that Andy will find a way to muck it up.

EXTRAS: "Extras Backstage" documentary, featurettes "The Art of Corpsing" and "Taping Nigel: The Gimpening," outtakes
PRICE: $29.98