The most deliciously scabrous skit on the mid-nineties HBO comedy series “Mr. Show” was “The Dewey Awards,” which skewered the sanctimonious trend of rewarding A-list Hollywood actors for their “brave” portrayals of the autistic, the mentally retarded, and other less fortunate types. Those rankled by the sight of Dustin Hoffman and Tom Hanks playing these parts, rather than real-life challenged actors, could gasp in private delight at this long-delayed castigation.

That sorry Hollywood trend was revived, in a way, when tall, blonde, impossibly gorgeous Charlize Theron won the 2003 Best Actress Oscar for her depiction of the decidedly un-gorgeous, scowling prostitute/john-killer Aileen Wuornos in Monster. Instead of casting an ugly or even mediocre-looking actress, the filmmakers won easy, Hollywood-approved points for making a beauty deglamorize herself. It’s not that Theron wasn’t superb in that part; it’s just a shame that her talent was only fully recognized after this “brave” transformation.

Yet all that said, to watch Theron in “Young Adult,” the new comedy from “Juno” director Jason Reitman and writer Diablo Cody, is to actually witness an actress in the bravest role of her career. At times, Reitman and Cody’s strenuous efforts to render Theron slovenly and hardened is irritating, a self-conscious stylistic trick that, again, begs the question, “Why didn’t they just hire a more average-looking actress?”

Theron’s character, Mavis Gary, is a thirty-seven year-old former prom queen, now eking out a living as a godawful teen-lit ghostwriter. Her profession is just the tip of her immaturity. She has nightly whiskey binges, passing out with her eyeliner and silicon cutlet-stuffed bra still on; when she wakes up, she nurses her hangover by chugging Diet Coke out of the bottle. If she’s not on a soulless casual date with some unworthy male suitor, she’s stumbling around her unkempt apartment in sweats and Hello Kitty t-shirts, scarfing down Ben & Jerry’s. OK, we get it: it’s a sweet-faced sweetheart acting mean and pathetic, just like Cameron Diaz in the ill-conceived “Bad Teacher.”

But once the plot kicks in—Mavis leaves Minneapolis for her small hometown to win back her old beau Buddy (Patrick Wilson), who’s married with newborn child—we see how complex and challenging, how out-on-a-limb, this role really is. Mavis isn’t your typical well-poised diva; she’s not snotty simply because she’s a bombshell and can get away with it. Her superiority complex is all-encompassing, but it is revealed mostly in tart, vicious little sound bites, in her variously aloof, dismissive and disgusted reactions to the impending reality of adulthood. Mavis is self-congratulatory without merit, eerily unaware of others’ needs and signals, an inveterate sulker—Theron smiles less here than she did as serial killer Wuornos—but there is clearly a backlog of pain and self-doubt welling beneath the surface, and we can’t wait to see it unleashed.

It is almost frightening how skilled Theron is at bringing this nearly unbearable character to life, how naturally she plays against dewy-eyed type and disappears into Mavis’ scarred, venomous mindset. It’s as if she can feel the audience starting to sympathize with Mavis’ dilemma and then, with one pitch-perfect delivery of a bitter rebuke, completely curdles that reaction.

“Young Adult” is not just Theron’s triumph, however. The screenplay is refreshingly short on the precious, attention-getting, writerly jargon that marred the last Reitman-Cody effort, “Juno.” The dialogue here is crisp, concise and wholly in touch with the tragicomic language of missed social cues, of failed connections (or in this case reconnections). And unlike most movies about arrested development, the filmmakers provide no easy uplift to Mavis’ stuck-in-high-school rut, no convenient moralism. The quandary of hitting one’s peak too early has seldom come off this bleak; for awhile, “Young Adult” plays like a lovelorn geek’s dark fantasy about what fate might befall a former teen dream. But the film, happily, is too sensible and low-key to resort to that mentality.

For most of “Young Adult,” we laugh at Mavis’ expense, as she gracelessly upends Buddy’s calm, suburban domestic life. In the most cringe-inducing scene—made appropriately jittery by cinematographer Eric Steelberg—Mavis beckons Buddy into a private room at his own baby-naming party, hellbent on stealing him away. “I’m a married man,” he says, to which Mavis coos, “That’s OK, we can beat this thing.” Minutes later, in jilted disgrace, Mavis bumps into Buddy’s laid-back, hippie-ish wife Beth (Elizabeth Reaser), accidentally spilling wine on herself; when Beth tries to apologize and offers to pay the cleaning bill, Mavis erupts, in front of an onlooking circle of guests: “Forget it. It’s silk—it’s fucked!” The tirade that follows provokes the most pained, ribald laughs of the season.

But the nicest surprise about “Young Adult” is that Reitman and Cody also gently mock the good-hearted small-town folks, so that even the most down-home viewers will share at least some of Mavis’ disdain. Buddy, as played with typical milquetoast appeal by Wilson, has certainly matured more than rude divorcee Mavis, but he’s so laughably bland—the type of guy that gets excited about the new Chipotle opening up on the strip mall—that you wonder what Mavis ever saw in him. (That question becomes more probing once it’s revealed that Mavis was a jock-worshipping slut for most of high school, but never mind small quibbles).

And Patton Oswalt’s Matt, a slightly less stunted, wiser man-child than his role in “Big Fan” (there, he lived with Mommy; here, he lives with his angry nerd sister), is not exactly called upon to be the angel on Mavis’ shoulder. Matt is somewhat of a cipher, the embittered high school loser that just wants to be loved. But he’s given a haunting backstory—a high school beating left him not only crippled but practically impotent—and a poignant strain of cynicism that allows him to connect—sort of—with Mavis. They’re two sides of the same damaged, self-pitying coin, and their scenes together are unexpectedly genuine, deliriously off-kilter. When Mavis barks at Matt, “Could you walk any slower?!” as he hobbles on crutches, he’s hardly a pushover about it; their barb-trading gives each of these doomed souls a long-dormant jolt.

“Young Adult” is not an easy film to watch; it’s often cruel in that it pulls no punches, even towards the nice characters, and it isn’t above punishing its star quite often. That Theron gives such an unpleasant role her all is brave enough, but unlike Diaz’ broad caricature in “Bad Teacher,” she imbues the role with a deep, unsettling sadness—a sadness that is no less than the character deserves. It’s a performance with a staggering—and yes, extremely brave—lack of self-awareness (watch the TRAILER).