Joan Rivers - where have you been all my life? That, at least, is what I thought after watching (twice) the documentary "Joan Rivers: A Piece of Work." Directed by Ricki Stern and Annie Sundberg, this convulsively funny movie takes an up-close and sometimes queasy-personal approach to its motormouth subject, who, when she's not making you howl with laughter (or freeze up in horror), brandishes her deeply held hurts, fears, prejudices, poor judgment and bad taste as if they were stigmata.

---

Once best known as Johnny Carson's permanent guest host (until he cut her dead) and for her disingenuous question "Can we talk?" (not you, silly), Ms. Rivers, now 77, began slipping down the pop-culture food chain in the 1980s after a series of calamities, including the 1987 suicide of her husband, Edgar Rosenberg. She continued to perform and to churn out books, among other pursuits. But for those who didn't know her way back when, she was little more than a red-carpet attack dog and plastic-surgery cautionary tale. She made you laugh (and cringe), but there was desperation in the sight of this former comedy savant who, between operations and peddling jewelry on QVC, lashed out at those whose celebrity eclipsed hers.

---

Whatever else you want to call her - and various names spring to mind while watching the movie - she seems an unlikely fit for Ms. Stern and Ms. Sundberg, whose documentaries include "The Devil Came on Horseback," about the atrocities in Darfur, and "The Trials of Darryl Hunt," about a wrongly convicted African American prisoner. However improbable, the match-up works, partly because the filmmakers don't approach her as either an entertainment industry untouchable or one of its casualties. They're sympathetic to her, but a touch cool, and certainly not fawning, which underscores their status as entertainment industry outsiders. This is just a look at a native in her natural habitat, sequins and feathered boas included.

---

The filmmakers probably didn't need to force their way into that habitat, because Ms. Rivers, who, with her daughter, Melissa, starred in a television movie about her husband's death, seems to have few boundaries. It isn't that she overshares on an obvious level - there are many biographical details that never even come up - it's that no other "Joan," no private self, seems to be lurking beneath the mask. Given the single-mindedness with which she pursues her career - Melissa Rivers likens that career to a second child - you have to wonder how any other Joan could have survived. It's no wonder that when Joan Rivers asks a radio host, "Who is the real me?" it feels like an honest question.

---

For 14 months Ms. Stern and Ms. Sundberg tagged along with their subject, racking up miles and yowzah moments in her relentless pursuit of fame, money, attention, love, kicks, masochistic thrills or whatever it is that makes Ms. Rivers, née Joan Alexandra Molinsky, run. Having lucked out with timing, they began shooting in 2008, the year before Ms. Rivers won the Donald Trump reality show "The Celebrity Apprentice." She was enduring a fallow period and fighting irrelevance any which way she could, which might account for why she agreed to this documentary. (She's also good friends with Ms. Stern's mother.) The filmmakers kept after her as she moved from airplanes to hotel lobbies, dressing rooms and onto the stage, where her cavernously wide mouth pours out invective, acid asides and jokes jokes jokes like water.

---

To their credit, the filmmakers don't try to make her look good, and while they omit some of her uglier routines, they don't (perhaps can't) ask you to love her. That's a wise move. Ms. Rivers may be a comic genius, but she's easier to admire from the distance of a movie seat and perhaps across the passage of time. An equal-opportunity offender, she has taken plenty of people down on her way up, including other women. Watching some of her nastier "Tonight Show" spots (which aren't in the movie), I find it hard to decide if her pokes at Elizabeth Taylor's weight are more painful than her self-lacerating jibes. Picking on Ms. Taylor was unspeakably mean. But Ms. Rivers's contempt for herself lasted longer: a lifetime, or so it seems.

---

Smartly, the filmmakers take on Ms. Rivers's own looks from the start, opening the movie with shots of her bare face - a shut naked eye, a thin line of mouth - as someone else greases it with makeup. It's only after she's put on this face that all these pieces come together in startling close-up. It's a nice metaphor for the effort it takes to assemble the product known as Joan Rivers, but the bluntness of the images and her gaze are disconcerting. Is she daring us to look, or begging? It's hard to know, and the filmmakers, who resist putting her on the couch, aren't saying. In the end, all you really know is that when she stands on the stage, it's as if she had tapped right into her id. It's a gusher.

---

"Joan Rivers: A Piece of Work" is rated R. (Under 17 requires accompanying parent or adult guardian.) Her language is so dirty that it's almost charming, when it's not appalling.

---