Joan Rivers never wants to be left alone. If a paparazzi sabotaged an intimate dinner date, she wouldn’t sock him in the jaw; she’d probably joke with him about her sagging 76 year-old boobs. For her, the only negative side of celebrity is that her many adoring fans, handlers and fellow comediennes (Kathy Griffin, for one) sometimes refer to her as an “icon,” which just makes her feel old.

“Don’t you think some people want to be loved for their humor, their soul, their vulnerability?” the radio DJ Dr. Joy Browne asks the habitually makeup-plastered Rivers. Her response: “I just want to be loved.”

And trying to be loved is essentially what Joan Rivers does for all 84 minutes of Ricki Stern and Anne Sundberg’s documentary, “Joan Rivers: A Piece of Work.”

It’s hard to tell if Stern and Sundberg admire Rivers for “telling it like it is”—which in this case means bitching in her stand-up routine that “a penthouse diva gets lonely, too”–or if they’re disgusted by her constant self-pitying outbursts, often shot with her plush Park Avenue furniture and ever-patient assistant surrounding her. Even if viewers don’t already hate Rivers, her notorious trashing of the fashion-impaired at red carpet events, her love of fur and facelifts, and that burnt-to-ashes, New Yawk harridan croak of hers, she is a trying centerpiece for a feature-length film.

But somehow, in the many composed moments between Rivers’ often funny, often sad, often unbearable bouts of insatiable self-promoting, Stern, Sundberg and crew show you the absolute dark side of fame—and it’s rendered even darker because Rivers herself can barely see it. She may decry fame for “choosing her” in one scene, sobbing over the cruel public lashings that can come with it. But it’s not long until we see her back in action, booking schmaltzy showcases, opening herself up to the same potential abuse, with no family besides her grown daughter, Melissa, to temper her. If you had to choose, during your twilight years, between the most sedentary, melanoma-scarred Florida retirement lifestyle or trying to keep up your hard-won celebrity long after its prime, you’d likely opt for the former after watching this movie.

No one can deny that Rivers helped pave the way for the blue female comics—Griffin, Sarah Silverman—bombarding the airwaves uncontested today. She was the first outspoken, angry female comic to nestle snugly in the limited, static world of television, becoming Johnny Carson’s virtual “daughter” in the 1960s after a multitude of appearances, and delighting bored, fed-up wives everywhere. Then, in 1987, Fox offered her a late-night show in direct competition with Carson, and he and NBC blacklisted Rivers. Her husband and collaborator, Edgar Rosenberg, was fired from the Fox show; Rivers, siding with him, was booted as well; and shortly after, Rosenberg killed himself.

Certainly Rivers had a right to be bitter from all this, and her humor became all the more tart-tongued in the years that followed. But instead of her brutal honesty cementing her as the next Phyllis Diller, she became a staple of the tackiest, nip/tuck-iest aspects of show biz: celebrity roasts, boastful memoirs, her own round-the-clock jewelry auction on television.

The irony of an accidental women’s lib pioneer resorting to such trash is scintillating, but at 76, Rivers seems all too willing to continue the downward spiral. It’s impressive that Rivers, like her younger counterparts, still has no shame about throwing out Holocaust jokes or shockingly explicit accounts of geriatric nether regions. But then why all the moping about not being a superstar, why all the false glamour? You can’t be a shopaholic princess and an edgy East Coast comic. You can’t trash A-list celebrities and then cry (as she does in multiple close-up scenes) about a few critics’ plastic surgery remarks. You can’t play yourself in a theatrical production (as she did for several months in 2007) and then balk about how you aren’t taken seriously as an “actress.” “Acting” isn’t Joan Rivers portraying Joan Rivers!

And watching Rivers try to regain her hipness, long ago betrayed by all the facelifts, through joking about taboo subjects not likely stemming from her actual life—anal sex, for instance—is just joyless. The most fascinating aspect of her comedy act is the rage she exhibits, more than the shock value of the humor itself. The film’s finest moment, when all of Rivers’ least attractive qualities—the forced crudity, the “poor me” histrionics—come to the fore, occurs late in the film, when she torments an audience member for walking out after a Helen Keller joke. “That’s not funny, I have a deaf son,” the offended man says, and he is met with a stream of expletives and a lecture on making light of the tragic. Her fury is strident and frightening. But afterwards, his protest is all Rivers can reflect on, and she seems genuinely troubled, genuinely aware that she can go too far.

Certainly, there are other times in “Joan Rivers: A Piece of Work” where the comic’s humane side comes through, and we do, despite all resistance, find ourselves sympathizing. No one would emerge unscathed from a husband’s suicide, and her loyalty to him is touching. As is her Thanksgiving speech about feeling grateful “every time I step into a limo,” her charity work to disabled fans, her sticking up—sort of—for her daughter when both appeared on “Celebrity Apprentice” last year (“I hope I get kicked off first, because I don’t want to look smarter or better than Melissa.”)

But the saddest character in this self-serving odyssey isn’t Joan herself: it’s Melissa, and you desire for a whole film centered on her. As she tries strenuously to retain a semblance of patience while forever cloaked in her mother’s shadow, we are shown a wounded, dark-eyed beauty, who doesn’t wear her plastic surgery well, whose touch-ups look ill-advised. She seems less embroiled by the trappings of fame as by the stigma of being made up in her mother’s image, which she seems far too principled for. She may not possess the acid wit of her mom, or aura of glamour, but she’s held on to her dignity.

Premieres Monday, April 26 at Tribeca Film Festival; released theatrically in NYC June 2010.