The moment of truth in Sarah Polley’s “Take This Waltz” occurs when two of its principals are buck naked. Showering in the locker room after a dopey swimming class, Margot (Michelle Williams), a married woman secretly tempted to stray, and her sister-in-law Geraldine (Sarah Silverman), more happily married but a recovering alcoholic, pontificate on the inevitability of domestic boredom. “New things become old,” Geraldine says, wistfully, as they stare at the equally nude, withered old ladies across the room.

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That terse yet honest four-word lesson is one that Margot, pushing thirty, desperately needs to absorb. Her doting but somewhat anal-retentive husband Lou (Seth Rogen), a chicken cookbook writer, has begun to bore her. The couple still partakes in cutesy games—she tickles him while he’s on a business call, he splashes cold water on her in the shower—but their rapport is more cozy and child-like than sexual. There’s a palpable disconnect between them; when Lou cooks an arduous three-course meal for her, refusing to break for sex, he sees that as devotion, while she feels only neglect. Similarly, when he takes her out for a fancy meal on their anniversary, Margot wants to talk and Lou doesn’t (“I want to eat good food, not catch up,” he mumbles).

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These are everyday problems, ones that any couple past the three-year point often experiences and overcomes. But Margot sees them as all-enveloping. Her unease is only exacerbated when the handsome, freewheeling Daniel (Luke Kirby), whom she bumps into twice, turns out to be her neighbor. Daniel is a struggling artist, somewhat of a loser; he doesn’t seem to have much of a life beyond flirting with Margot. But he’s different in ways that excite her. He operates a rickshaw for pay, he likes watching the waves break at sunrise, and he’s not afraid to call Margot on her hypocrisies, something her gentler husband would never attempt. After graphically sweet-talking her in a café (“you smell like sweetness and fuck”), Daniel has hooked Margot; her only defense is to coyly schedule their inevitable tryst for “thirty years from now,” and of course Daniel works overtime to break down that resistance.

As with her last film, “Away from Her,” Polley has an undeniable gift for bringing humor out of almost unbearably sad situations. In that film, she evoked the absurdity, as well as the heartbreak, of having to accept that your Alzheimer’s-stricken wife has become a stranger, and here, she does the same with the touchy subject of cheating and broken marriages. Part of the time, you are angry with Margot for running away from a basically decent companion, for acting out a fantasy that is at best fleeting. But at other times, you understand completely what’s fun and fulfilling about, say, sneaking off to an amusement park with a would-be lover (in that sequence, Polley achieves the unthinkable: she turns the peppy, corny Eighties new-wave hit “Video Killed the Radio Star” into a melancholy symbol for loneliness.)

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Polley has cast “Take This Waltz perfectly.” It’s become a little tiresome to watch Michelle Williams play unhappily-married women, but Margot is a sunnier, sillier character than the frowning, no-nonsense wretch she portrayed in “Blue Valentine.” Watching Margot’s baby-talk back-and-forth with Lou, you see a playful, unrestrained side of Williams that had never been exposed before. You also see the pain beneath the comic banter. Even in the midst of lust for Daniel (played with the right blend of suaveness and off-putting aggression by Luke Kirby), Williams shows just enough guilt and indecision to convey Margot’s loyalty to Lou. Even when stooping very low in the film’s third act, she’s never a monster–only pitiable.

Rogen gives his quietest, subtlest performance yet, proving his mettle for drama. It’s chilling watching his character switch, with no warning, from buffoonish roughhousing to hypersensitivity. And Silverman is a marvel as the impetuous, yet wise, sister-in-law.

Polley frames scenes beautifully; her visual metaphors—squabbling characters sulking on opposite sides of a porch window, a blurry woman slowly coming into focus, the daunting shadow of a lothario as he overtakes a woman on a beach—never seem forced or cloying. She’s made a movie that takes on an often irreparable, yet frighteningly common problem in society—mistaking impulsivity for immortality, and winding up more alone than you ever imagined—but the tone is never hectoring, never puritan. “Take This Waltz” just happens; it’s a staggeringly natural film.

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