WITH HER strangely modern, saucily jutting jaw line and Southern-belle sweetness masking a crackling intellect, Becky Sharp is portrayed perfectly by Reese Witherspoon as the pretty, opportunistic and contradictory heroine of William Makepeace Thackeray's "Vanity Fair." In an engaging, propulsive performance as the strong-willed, social-climbing English governess who aspires to enter high society by any means, Witherspoon moves director Mira Nair's version of Thackeray's social satire forward at a good clip, making Becky's rising and falling fortunes an intensely watchable spectator sport.

Unfortunately, Nair's film doesn't so much end as fall off a cliff, the ultimate victim of viewers' heightened expectations that this briskly paced story will take them someplace -- other than around the block in a horse-drawn carriage.

Set in the early years of the 19th century, when the rungs of English society were even more sharply defined by class (on the upper echelons of which Thackeray cast his jaundiced gaze), "Vanity Fair" is not the kind of story with a conventional hero, and Becky's behavior, which might charitably be called expedient, is far from something to root for. Still Witherspoon can't help making her heroine far more likable than Thackeray's ever was. Which means that, when all is said and done, we want more for her than she ever deserves, or gets.

Trading on her beauty, cleverness and what one character calls her "talent for life," Becky rises from near poverty to comfortable -- if borrowed -- wealth over the course of the film, thanks to such benefactors as a smarmy nobleman named, appropriately enough, Steyne (pronounced "stain"). Played by Gabriel Byrne with a jaded half-smirk, Steyne is, on the one hand, a kind of villain, seeing as he ultimately demands something unseemly from Becky for all his social introductions and unsolicited financial assistance. And it is Steyne, after all, who arranges for the military posting that puts Becky's handsome if rakish husband, Rawdon (James Purefoy), in harm's way after Rawdon walks in on Steyne attempting to collect his due.

Yet Steyne is also a stand-in of sorts for Thackeray. An observer of hypocrisies who is both of and above the world he criticizes, he mocks the upper crust even as he partakes of its nourishment.

If Steyne is the bad guy, however, Becky isn't quite the good girl we -- or Nair and Witherspoon -- might like her to be. For one thing, she's an indifferent mother, and her palpable pain when her son is taken away from her late in the film is tempered by our memory of how little she seemed to care for him when, for example, he was taking his first steps.

Thanks to Witherspoon, we may find ourselves wanting Becky (a true survivor) to make it, at the same time that we feel disapproval for the ways in which she does, and the toll those ways take on our esteem for her.

Where "Vanity Fair" succeeds is in its indelible depiction of the lively central character, who makes us both care for her and disapprove of her flawed moral outlook. Yet this is also the movie's failing. Shouldn't a character this appealing and this smart be able to learn the error of her ways over the course of her journey?

That she doesn't isn't really Thackeray's fault so much as Witherspoon's. She is, in a way, too good. After all, the author doesn't care all that much for Becky, except as a literary device with which to make his sharp points about the haves, the have-nots and the wanna-haves. The actress, however, has gone to such lengths to breathe life, spirit and soul into Becky that it only seems reasonable for the audience to hope for a redemption for the character that never comes.

VANITY FAIR (PG-13, 140 minutes) -- Contains brief partial nudity, a mild boudoir scene, scuffling and images of war dead. Area theaters.

Becky (Reese Witherspoon) climbs her way to the top of the social ladder with husband Rawdon (James Purefoy) in "Vanity Fair."