Despite the celestial similarities, however, A Life has higher ambitions than either of these two films: nothing less than justifying the ways of God to men and women, and those of the filmmaker to his benighted creations. Stylistically, too, Boyle and Hodge set themselves an inordinate challenge: combining such disparate genres as the screwball comedy, film noir, the road movie, and the musical, sometimes all in the same scene, and balancing their often grave themes and graphic violence with a tone of near-hysterical whimsy. Sometimes A Life simply tries too hard to be madcap and strange, whereupon it falls as flat as does Stanley Tucci early on in the film's ill-conceived William Tell sequence. More often, though, it reaches heights of divine aspiration.

What sustains A Life through the low points is its sparkling cast. As Robert, a janitor with dreams of writing a trashy bestseller (whose premise sounds as bankable as most that are made into movies, including this one, despite everyone's disparagement of it as "obvious"), Ewan McGregor evinces a sweet naïveté on the near side of simple-mindedness. With a bad haircut and a patterned shirt, he's a cross between Dudley Moore and Malcolm McDowell, suggesting a sweetness just about to turn, a softness with a glint of razors underneath.

Replaced by a robot, Robert confronts his boss, the soulless corporate magnate Naville (it rhymes with DeVil), to whom Ian Holm gives a somber, steely edge. When the meeting collapses into some brilliant sight gags (Boyle's visual wit is reminiscent of the Coen Brothers, especially in Raising Arizona) involving beefy security guards, an errant handgun, and Robert's dogged mechanical replacement, Robert kidnaps (a crime increasingly posed in movies as an alternative to unemployment) Naville's spoiled daughter Celine (Cameron Diaz, leaving Julia Roberts in the dust and then some). He speeds out to the desert without a clue what to do next.

Not that he needs one, for the destinies of both kidnapper and victim are being manipulated, however ineptly, by two angels on probation for failing in their mission to promote true love on earth. Jackson (Delroy Lindo, a little uneasy as an emissary of Heaven) and O'Reilly (Holly Hunter hamming it up as a bimboish tough cookie with a touch of the Terminatrix) have been summoned to the office of Chief Gabriel (Dan Hedaya) and given one last chance to retain their heavenly status. They must effect the union between this most unlikely couple or be marooned forever on earth.

The ground rules of their intervention seem fast and loose -- call it winging it and desire. After manipulating the kidnapping, the angels set the couple up in situations of increasingly outrageous danger and absurdity. The simplest schemes work best, as when, in a sublime inversion of the karaoke scene in My Best Friend's Wedding, Robert and Celine perform a rousing production of "Beyond the Sea" in a redneck bar. Some situations don't make any sense at all -- for example, the entire ending.

No matter. Diaz and McGregor irradiate the screen with their ardor, tenderness, and good humor. Initially a snide bully, Celine comes across as even more adorable than when Diaz plays a nice girl. The melting of her toughness gives backbone to the wimpy Robert, and it becomes clear that any ending to the movie will be a happy one. Along the way there are plenty of earthly delights to be savored, such as hilarious bit parts by Maury Chaykin as a backwoods weirdo named Tod and Tony Shalhoub as a bartender who serves as Robert's voice of reason. Then there are ravishing, sun-infused landscapes and gaudy interiors reminiscent of high-quality kitschy postcards. A Life has the courage to be a film less ordinary, and though its reach exceeds its grasp, it shows what Heaven is for.