Currently available on DVD: "Joan Rivers: A Piece of Work"

Fear, as defined in the bawdy and surprisingly trenchant documentary,Joan Rivers: A Piece of Work, is a blank calendar. There is a riff to do on the terror she finds in all those little empty squares. But by then you know it's no laughing matter. Work is life; not working might as well be death.

With a mix of moments like that — as poignantly revealing as they are entertaining — along with TV clips, many of them classics, and months spent following the comic through the long days and nights of her 75th year, filmmakers Ricki Stern and Annie Sundberg have managed to capture the "Can we talk?" comic in all her funny fury.

She is, to put it mildly, still brazen after all these years. When talking of being a sort of birth mother for the current caustic strain of female stand-ups like Kathy Griffin or Sarah Silverman, she explodes, "… them." Classic Rivers — half joke, half line in the sand, all R-rated.

But then, she has been crossing lines of decorum from the beginning, as the film deftly reminds. After all the dreary red carpet duty of recent years with daughter Melissa, it's easy to forget that Rivers was a groundbreaker. Using clips and conversations, the filmmakers go back through the pre-Roe vs. Wade early years when she made abortion a bit in her act — euphemistically when she was ripping through the TV talk show circuit, and unabashedly when she was onstage — despite being told it was career suicide. Nothing is off limits if it gets a laugh, and that's a lot as a wall in her apartment, lined floor-to-ceiling with carefully organized files of all the jokes she's ever written, testifies.

The story, though, begins with Rivers' 75-year-old face. The woman who has long been a plastic surgery punch line is, for all practical purposes naked, the camera zooming in on a makeup artist's careful ministrations — broken veins covered, eyebrows penciled in, lipstick put on a mouth cut tight by scalpels. In scenes like this, the filmmakers prove adept at exposing her reality — harsh under bright lights — whether Rivers chooses to talk about it.

Her unrelenting ambition and its often heavy costs, as much as her humor, becomes the documentary's central theme. In one particularly telling moment daughter, Melissa, who with her mother was cast on Celebrity Apprentice in 2009 while the documentary was shooting, explains that while Rivers might think she wants Melissa to win, it's just not in her genes to hold back. That's exactly how it plays out with Melissa booted and Rivers going on to claim the title.

The high price is the subtext as she remembers Johnny Carson. He would give Rivers her big break in 1968, and she became his permanent guest host. But he cut her off completely when she launched her own late-night talk show on Fox in 1986, and it still brings her to tears that he never spoke to her again.

On the surface, Rivers seems an unlikely choice for frequent collaborators Stern and Sundberg, who've made social justice stories their métier: from genocide in Darfur to a North Carolina man wrongly convicted of a murder-rape that cost him 20 years in prison. What does echo their earlier efforts is the presence of a strong central character and the film's quiet statements about the relative value of a life in a celebrity-obsessed society where even 15 minutes of fame is so avidly desired.

The film's weakness is the perspective on Rivers — it is single-focus and primarily hers. There are no detractors among the many interviewed, though with more than 40 years in the entertainment business there must be a few people besides Carson that Rivers rubbed the wrong way.

That singular focus is also a strength, with her candor as untempered as her ambition. She shows up for rehearsal for her one-woman play one day right after a cosmetic drive-by that has made her face a puffy pin-cushion, newly pumped with fillers and smoothers. Not a pretty sight. But the scene is not so much about the play as the fears that fuel her 18-hour days: that she won't look good for the show, and that some day, there won't be a show.

Among the few personal notes in the film is her annual Thanksgiving dinner for family and friends in her New York apartment, a luxe rococo-Victorian mash-up of silks and brocades. But the holiday, like any personal life in the traditional sense, seems little more than a brief interlude from the thing she really craves.

Best is watching her work the dive clubs beaten down by decay. In one, a bar stool on the stage is literally coming apart at the seams, and soon becomes a prop for her jokes; in another, there's a heckler she takes apart at the seams, an even better prop. She is by turns blue, bitter, hilarious, unbroken; a Hollywood-style portrait in infinite ambition. In that role, Rivers is unforgettable.