May 17, 2002 | You may be happy to know - and, then again, you may be disappointed to see -- that Paul and Chris Weitz, the sibling filmmakers who gave us American Pie, steer clear of doing rude things to baked goods in their latest opus. Instead, they've whipped up something even tastier and funnier by mixing prickly wit, droll dialogue and cunning sentiment, then adding just a dash of edgy, messy tragicomedy as a pungent garnish.

The result is About a Boy, a seriously funny movie about a 38-year-old layabout who's dragged kicking and screaming into adulthood, and the 12-year-old misfit who does much of the heavy lifting while weighed down with his own emotional baggage. It's a match made in movie heaven, and it's all the more amusing and affecting because of the spot-on perfect casting of Hugh Grant and newcomer Nicholas Hoult.

Grant -- sporting a spiky new 'do that prevents him from falling back on familiar, floppy-haired mannerisms -- is introduced early on as Will, a shallow cad who frankly acknowledges his own selfishness: "I'm on my own. There's just me. I'm not putting myself first, because there isn't anybody else there." He doesn't have to work, thanks to royalties from a novelty song written long ago by his late father. (It's a chipper Christmas tune, Santa's Super Sleigh, which, of course, Will detests.) So he spends most of his waking hours on… well, on not much of anything, really, except eating, drinking, buying stuff and watching lots of TV.

It's a simple life, Will concedes, but a fulfilling one.

Every so often, however, even a self-absorbed slacker like Will feels the need to do something good for someone else, provided it doesn't require much in the way of time, money, effort or personal involvement. He praises himself for volunteering to work at a soup kitchen, even though he never actually gets around to serving food there. And he proudly recalls doing telephone solicitation work for Amnesty International. Trouble is, he spent most of his time there chatting up single women on the other end of the line.

Will isn't drawn into genuine selflessness until, ironically enough, he attempts something that, even by his standards, is deucedly cheeky, if not downright shabby. After a brief, fortuitous fling with a divorced mom, Will deduces that single mothers are the perfect partners for short-term, highly sexed romances. So he pretends to have a 2-year-old son, and joins a single-parent support group, all for the purpose of stalking fresh prey. He quickly attaches himself to Suzie (Victoria Smurfit), an available beauty. Through Suzie, however, Will becomes even more closely entwined with Marcus (Hoult), the troubled young son of Fiona (Toni Collette), an unreconstructed hippie who's given to singing '70s pop tunes, proselytizing for vegetarianism and, with alarming frequency, wallowing in suicidal depression. When Suzie, Will and Marcus find Fiona has taken an overdose of sleeping pills, they rush her to a hospital. Suzie drifts out of the plot. But Will and Marcus, despite Fiona's initially strong misgivings, slowly form an unlikely friendship.

Fiona's attempted suicide comes as a jolt -- though not, strictly speaking, as a surprise -- but the only really quease-inducing moment in About a Boy comes later, when Fiona learns Marcus has been spending most afternoons at Will's apartment, watching TV and binging on junk food. She wants to know why a 38-year-old man would take such an interest in a 12-year-old boy, and the movie must pause -- briefly, unavoidably -- to ask the obvious questions.

The relationship is purely innocent -- well, OK, not exactly pure or innocent, given Will's status as a Bad Example, but altogether platonic -- yet Will must have a moment or two of enraged fluster as he defends himself against Fiona's not-so-veiled accusations. Truth to tell, he opens his door to her son only because Marcus won't stop ringing his bell. Opening his heart to the boy requires more of a commitment than just buying him new sneakers or cool CDs. And that's precisely the kind of commitment Will has always avoided as fervently as Superman dodges Kryptonite.

Other painful real-world concerns intrude on the deceptively light and bright fun throughout About a Boy. Marcus, who shares the job of narrator with Will, is brutally bullied at his new school for being "different" (i.e., more like his mother than his classmates). Will takes a good, long look at himself, and doesn't particularly care for the view. And Fiona, compellingly played by Collette as a woman turning into a forlorn ghost of herself, never appears very far from making another attempt to shed her mortal coil.

Doesn't sound like a laugh riot, right? But never mind: The serious stuff doesn't merely interrupt, it actually enhances the funny business, making for an exceptionally fine and richly entertaining film that only appears to be facile and featherweight.

Grant, a master at suggesting unplumbed depths in the shallowest blokes, charges his winning, wisecracking performance with alternating currents of farcical physicality -- note Will's wide-eyed reaction to a friend's new baby -- and wistful discontent. Better still, Grant brings out the best in his affecting yet unaffected co-star, Hoult, who is refreshingly free of kid-star cutesiness while conveying how arduously challenging childhood can be.

The Weitz brothers and co-screenwriter Peter Hedges are smart enough to remain faithful to their source material, a marvelously clever novel by Nick Hornby, whose High Fidelity also was served well in cinematic adaptation. (A funny thing: Stephen Frears, the British director of Fidelity, transferred that novel's plot from London to Chicago, but the American-born Weitz guys keep Boy where Hornby originally placed it.) Like Fidelity, Boy is the sort of book you should never read in a public place, unless you don't mind being stared at while you laugh out loud as you peruse the more hilarious patches. But you won't have that sort of problem with the movie version of About a Boy, because, in a theater, you'll be surrounded by folks who'll be laughing just as loud, if not louder.