It's not easy being mean, as Oscar-winning screenwriter Diablo Cody's latest project, "Young Adult" — directed by her "Juno" collaborator, Jason Reitman — goes about illustrating with an intriguing, unsettled blend of pity and pitilessness.

Thirty-seven-year-old borderline alcoholic Mavis Gary, handled with unsentimental aplomb by Charlize Theron, may have left her hometown of Mercury, Minn., for the exotic urban center of Minneapolis. But emotionally and psychologically, she's still in high school, honing her worst social instincts to a needle point. Ghost-writing young adult novels for a living, Mavis is defined by what "The Descendants" author Kaui Hart Hemmings referred to when describing someone as having "carried her juvenile meanness into her adult life."

The mission, as Mavis sees it and is only too happy to accept, is simple: Return to Mercury wielding a dubious excuse (something about a real estate transaction); ignore her parents as long as possible; reconnect with her sweet-natured high school boyfriend, Buddy (Patrick Wilson); and bag him for keeps. Buddy is married (winning Elizabeth Reaser plays his enviable wife, the special-ed teacher) and has a newborn. But Mavis knows what she wants.

Narratively this is a simple tale, hinging on the misfit-musketeers alliance of Mavis and an unpopular former classmate, played with lovely tact and shrewd timing by Patton Oswalt. True to Cody's darker comic instincts, this character — crippled for life by "those jocks" in a hate-crime incident back in high school — is identified by Mavis as "that hate-crime guy." Later she gripes to her confidant with the crutches: "Could you walk any slower?"

This is train-wreck human misbehavior all the way. You watch "Young Adult" horrified by Mavis' delusions. Cody has created a role offering Theron a lot of room and plenty of casual venom.

Yet Mavis isn't so different from the small-minded vampire seductress played by Megan Fox in "Jennifer's Body," also written by Cody. The anti-heroine in "Young Adult" is held up for our voyeuristic amusement and, finally, our contempt. Her impulses are patly explained away by the carefully timed revelation of a painful secret. Theron nails the specifics of her character, all the distress signals, from the nervous hair-pulling to the relentless default sarcasm. I wish, though, Reitman (whose own instincts are to find ways to tenderize dislikable people or situations) had found a way to give "Young Adult" more of a motor. Cody's a fast wit, and "Young Adult" is not "Juno," which proves she can write in more than one key. Yet Cody would likely acknowledge she's working through her own contradictory feelings toward her protagonist — and that she may have been a draft or two away from shaping those feelings into a terrific black comedy, rather than a pretty interesting one.

'Young Adult' -- 2 1/2 stars

MPAA rating: R (for language and some sexual content)

Running time: 1:34

Opens: Friday