Movie stars, as they get a bit older, tend to retain their essential disposition -- the quality that made us fall for them in the first place. (Often, they retain it to the point that they feel locked into it.) The Hugh Grant who stars as a rakishly self-centered bachelor in About a Boy, however, is a very different animal from the sweetly abashed overgrown schoolboy who pined and stammered his way through ''Four Weddings and a Funeral'' and then, in an even more remarkable performance, drew on the same charmed tizzy of will-somebody-please-toss-me-a-life-jacket distress when he appeared on ''The Tonight Show'' to apologize for his naughty offscreen behavior. That earlier Hugh Grant was a walking contradiction -- an uptight dreamboat, held hostage by the very manners that lent him such an endearingly civilized sex appeal. He was anxious to the point of pain yet also terribly, touchingly sincere. He may have been the last leading man who could get away with acting like a nervous wreck.
The first thing you notice about Grant in ''About a Boy'' is that he looks older. The second thing you notice is that it looks good on him. The pillowy cheeks are flatter and a bit drawn, and the eyes that used to peer with ''love me'' cuteness now betray a shark's casual cunning. Everything about him is leaner and spikier (including his hair, which has been shorn and moussed into a Eurochic bed-head mess), but it's not just his surface that's more virile; the nervousness is gone, too. Hugh Grant has grown up, holding on to his lightness and witty cynicism but losing the stuttering sherry-club mannerisms that were once his signature. In doing so, he has blossomed into the rare actor who can play a silver-tongued sleaze with a hidden inner decency.
In ''About a Boy,'' Grant draws on, and deepens, his enjoyable performance as the seducer boss in ''Bridget Jones's Diary'' to play a new species of highly self-aware cad. His character, Will Freeman, is a London womanizer who is in the absurd, if enviable, position of not having to work. He lives off the royalties from ''Santa's Super Sleigh,'' a Christmas jingle composed by his late father, and this leaves him free to pursue his grand passions: watching TV, watching videos, playing ''sports'' (i.e., billiards), and chasing women he has not the slightest intention of settling down with. ''About a Boy'' is based on the 1998 novel by Nick Hornby, and with Grant reciting Will's acid thoughts to us in voice-over (it's hilarious to see him confront his friends' young child and then hear what Will really thinks about the situation), the movie sticks much closer to Hornby's drop-dead confessional tone than the film version of ''High Fidelity'' did.
Will, in a sense, is lying to everyone he meets, because the secret of his life is that he belongs, quite contentedly, to a club of one. He's the ultimate refinement of the modern bachelor, slicing his time, like a male Bridget Jones, into units (haircut, two units; sports, three units) to be filled however his pleasure dictates.
That is, until he gets the bright idea of scoring a date by infiltrating a support group for single parents and pretending to be one of them himself. Through complications arising from this charade, he is drawn into the lives of Megan (Toni Collette), a not-so-ex-hippie with a chip on her shoulder as heavy as her depression, and Marcus (Nicholas Hoult), her 12-year-old son, who's a sensitive outcast at school. Actually, the on-the-make Will wants nothing to do with these two. (He wants to date Megan's neighbor.) But when Megan is rushed to the hospital after a failed suicide attempt, he finds himself in the grudging position of playing comrade to the isolated boy.
Each day, Marcus comes over and rings the doorbell, and he's in such dire need of an authority figure who isn't a basket case that Will, with nothing but time on his hands, takes pity on him and starts to occupy the role. There's a certain dampening of excitement when you realize that ''About a Boy'' isn't going to be a contempo ''Alfie'' but, rather, one of those movies like ''Big Daddy'' in which a screwup is redeemed by his camaraderie with a kid. Yet part of the picture's funky sweetness is that codirectors Paul and Chris Weitz, aiming a bit higher than they did in ''American Pie,'' do more than transform Will from a trampy hedonist into a nice guy. They trace the careful, step-by-step process through which compassion starts to feel more real to him than his shiny, empty mirror of a life did.
The title, of course, refers to both of the main characters. If Grant himself has rarely seemed less boyish, newcomer Hoult, who looks like an enthusiastic owl, creates the most touchingly unguarded edge-of-adolescence nerd in ages. It would have been nice if the big school-performance scene at the end were as inspiring a heart-tugger as it wants to be. Alas, it's a mixture of the heroic and the embarrassing, in no small part because the sight of Hugh Grant singing ''Killing Me Softly,'' a song that's perfectly respectable to like, turns out to be not nearly as sublime as Grant reciting the lyrics to ''I Think I Love You,'' a song that gets no respect whatsoever. For a moment, the new Hugh Grant made me miss the old one.