‘Extras’ offers more than the usual

“Extras,� which premieres Sunday on HBO, is Ricky Gervais’ follow-up to “The Office,� the BBC series that made him famous and, I suppose, rich. The series, which focuses on the world of the uncredited many who fill the background and corners of a moving picture, is at once more modest and more ambitious than its predecessor; more focused on detail and yet more expansive. It is also excruciatingly funny, with an emphasis on excruciating.

Gervais plays Andy Millman, a man in his early 40s who, five years before the show opens, has quit work to try to make it as an actor, in all of which time he has not emerged from the background, or even spoken a line of dialogue -- at least not one that anyone has heard.

(It does not help that he has the world’s worst agent -- played by Stephen Merchant, Gervais’ writing and directing partner on “The Office� and “Extras� -- who, in trying to explain why he’s never landed Andy a job, offers, “I’ve got a feeling it may be your shape; if you insist on remaining a blob could you at least get a tan?�) It’s never discussed why Andy has taken this step, or why he thinks he might make a career of it, and there is no back story. He is clearly related to David Brent, the creepy boss with show-biz ambitions Gervais played in “The Office,� and with whom he shares a certain energy and repertoire of facial expressions. But he is smarter -- better-informed about the world, and not only more aware of himself but aware of other people, which means that he is also kinder. But because he is also socially inept, his kindness can often turn cruel.

As in “The Larry Sanders Show� and “Curb Your Enthusiasm,� two obvious influences on “The Office� and “Extras,� the series features appearances, and not just cameo appearances, by celebrities playing themselves: Samuel L. Jackson, Ben Stiller, Kate Winslet and Patrick Stewart, along with British soap opera star Ross Kemp and game show host Les Dennis, most of them seriously subverting their public image. Stewart is especially hilarious describing a film, which consists mostly of women losing their clothes. Winslet offers instruction on how to talk dirty on the telephone.

Unlike “The Office,� in which there were tensions and story lines that begged for resolution, there is little obvious movement in “Extras,� no great story arc: At first, as we visit Andy and his slightly dim best friend Maggie (Ashley Jensen) on one movie set or another, it seems the show is merely about Andy’s attempt to get someone to give him a line. But the six-part series, which gets stronger as it goes along, ends in a way that brings everything that preceded it into a new focus. You realize you’ve been watching a portrait of a friendship, the friendship between Andy and Maggie, which will -- thank heaven -- not blossom into love, though both are lost and lonely. Jensen’s performance is astonishingly deep and true -- you can hardly believe it’s on a sitcom.

But the series itself is deeper than you expect. Unlike most, if not all, American TV shows, “Extras� accepts sadness as a condition of life, not a transitory effect to be obliterated in a fourth-act blizzard of good feelings, but something that can only be kept at bay. That awareness is what pushes it toward greatness.

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‘Extras’

Where: HBO

When: 10:30-11 p.m. Sunday.

Ratings: TV-MA (may be unsuitable for children under the age of 17)

Ricky Gervais...Andy Millman

Stephen Merchant...Agent

Ashley Jensen...Maggie Jacobs

Executive producer: Jon Plowman. Writers and directors: Ricky Gervais and Stephen Merchant.