December 12, 2003 | Don't look now, but Bobby and Peter Farrelly - the cheery vulgarians who gave us Kingpin, Dumb and Dumber and There's Something About Mary - have gone all softhearted and semi-respectable just in time for Christmas.

Telltale signs of their transformation already were apparent two years ago, when the filmmaking siblings unveiled Shallow Hal, an oddly affecting farce that relied relatively little on gross-out gags and scatological humor (i.e., Farrelly trademarks). And now we have the sweetly funny lunacy of their Stuck on You, an even kinder and gentler comedy about the misadventures of adult conjoined twins.

In the tricky lead roles of Bob and Walt Tenor, joined-at-the-hip co-owners of a Martha's Vineyard diner, Matt Damon and Greg Kinnear double your pleasure, double your fun, with performances that winningly balance heart and humor. But wait, there's more: The illusion of their attachment is sustained with a minimum of special effects, and a maximum of deft physicality.

Walt (Kinnear), the more outgoing of the pair, often acts on local stages, even while brother Bob (Damon) prefers to remain literally in the shadows. Bob's anxious avoidance of the spotlight can make for awkward moments - especially while Walt is appearing in a one-man show - but when Walt decides to try his luck in Hollywood, Bob reluctantly goes along for the ride. Not surprisingly, complications ensue.

At once warm-hearted and sharp-witted, Stuck on You freshens a formulaic fish-out-of-water story with generous sprinklings of sentiment and absurdity. Back home in Martha's Vineyard, Bob and Walt are nonchalantly accepted by supportive friends and neighbors (many of whom, it should be noted, have their own colorful quirks). In Hollywood, the brothers must muddle through without their support system - but, since it is Hollywood, their condition rarely elicits anything more than a second glance or a double take.

Morty O'Reilly (Seymour Cassel), Walt's seedy and near-senile agent, complains that casting "Siamese twins" will be, at best, problematical. But Bob's main worry is that he'll make a bad impression on a beautiful e-mail pen-pal (Wen Yann Shih) who just happens to live in L.A. Meanwhile, Walt lucks into success as the co-star of a prime-time TV drama with - are you ready for this? are you sitting down? - Cher.

Cast as herself - or, more precisely, as an inspired self-parody - Cher reminds us of her considerable gifts as a sharp, straight-faced comedienne while fearlessly making herself the butt of the movie's most deliciously nasty satirical jokes. Likewise, Eva Mendes (Out of Time, The Fast and the Furious) does a spot-on job of playing Eva, a ditzy starlet whose generosity of spirit is surpassed only by her surgically enhanced bust size.

The Farrellys still have a lot to learn about such niceties as pacing, camera placement and matching shots. (When Bob and Walt converse during a snowfall, it's all too obvious that the scene was filmed during varying climatic conditions.) But never mind: They're bound to keep you ho-ho-hoing with the most surprisingly appealing comedy of the holiday movie season.