CANNES - There are few faces -- individual, honest-to-God faces -- in the movies today quite like that of Marion Cotillard, her startling beauty assembled from oddly sized, quizzical features that mightn't hang quite right on anyone else's bones. She looks like no one else, and yet never quite resembles herself on screen either: it's a face that different angles and contexts can turn from silken to sallow, hunter to hunted, goddess to guttersnipe. It is, in other words, the closest thing to a character actor's face that a cover girl can have.

Small wonder, then, that Cotillard is the actress to whom those sober bastions of back-of-the-head realism, Luc and Jean-Pierre Dardenne, have turned to headline their very first star vehicle. (Some may cite Cecile De France in "The Kid With a Bike"; if she is a star, she didn't have her film's undivided attention.) The brothers have always had a collective eye for great faces -- they're the ones who made Emilie Dequenne and Jeremie Renier, after all -- but their becomingly modest tales of urban-fringe survival have never seemed especially suited to famous ones. Cotillard's singular radiance, however, makes her the ideal lead for the Dardennes' latest hard-luck snapshot, "Two Days, One Night" -- in which a woman's personal charisma is the only thing keeping her from the breadline.

If "Two Days, One Night," for all its typically tender-tough virtues, feels uncharacteristically contrived for a Dardennes joint, its leading lady cannot be held to blame. Cotillard sells every sweat-stained note of Sandra, a careworn working-class mother of two in the Belgian industrial town of Seraing -- crucially never playing it as a guise that needs selling in the first place. There's no ostentatious deglamming here, no crumpled teeth or effortfully distressed hair. Clad in clean, sorbet-bright tank tops, the distinct tremor in her voice intact and unaccented, Cotillard instead gets to work on Sandra's long-brewing inner exhaustion and (as we soon learn) her diagnosed depression, nailing each insecure or irritable tic with unshowy exactitude.

It's with a mixture of slack defeat and flinty pique that she receives the latest bad news: her job at a solar-panel factory has fallen prey to downsizing. Following a poll of her 16 co-workers, asked to choose between retaining her job or their €1000 bonuses, self-interest has carried the day -- with only two people taking her side. After learning that her manager unduly influenced the vote, it's all Sandra and one of her sympathizers can do to a secure a second secret ballot the following Monday -- giving her the weekend to personally visit the 14 naysayers in the hope of talking them round.  

It's a tight, urgent premise, crisp and clean as an on-the-clock western -- and entails an rare degree of narrative construction from the brothers, usually content to follow their characters to more ambiguous ends. Every face-off between Sandra and a wary colleague is its own tense individual drama of negotiation and resistance, each one culminating in a mini-climax that either mildly releases or further applies the pressure on her skinny shoulders; there may as well be an on-screen scoreboard to keep count of the contrite yeses and stubborn nos as they accumulate. The brothers' keen skills of social observation are also well-served by the door-to-door structure, as they subtly delineate the significant range of home environments -- differing according to class, race and any number of personal variables -- enjoyed across one set of co-workers.

That tidiness turns out to be a mixed blessing. After the Dardennes' last film, "The Kid With a Bike," arguably set the stakes a little lower than usual, the compression of "Two Days, One Night" makes for more nervier, more compelling viewing. (The bright, sun-baked atmosphere of "Kid," however, is here to stay.) Yet there's also something exercise-like about this staggered, short-cut narrative that, however enjoyable, isn't entirely authentic, while the film's politics are liberally jerry-rigged in a way that their more organic studies of the institutional indignities visited upon Belgium's less privileged classes rarely need to be.

Even before some unwieldy second-half lapses into outright melodrama (one particular instance of rock-bottom behavior glossed over in bizarrely short order) "Two Days, One Night" has achieved a curious feat for the auteur duo: rousing and touching as it so often is, it may be the first Dardennes film that I don't entirely believe. It's Cotillard's unwavering conviction, and her just-right relationship with warmly empathetic onscreen husband Fabrizio Rongione, that holds our heart through the film's least credible spells. Thanks to her, the film earns a niftily reversed ending that counts as a gut-punch and an air-punch at the same time. Of the many possible outcomes presented by a Cotillard-Dardennes collaboration, few would have bet on the star -- and her storied, extraordinary face -- bringing her directors back to reality.