One of the most incisive—and also one of the most non-objective—films in recent memory about adult relationships, "Take This Waltz" astonishes in its raw, unaffected intimacy. 28-year-old Margot (Michelle Williams), an aspiring writer, briefly runs into Daniel (Luke Kirby) while on assignment in Louisbourg, Nova Scotia, then meets him again on the flight back to their hometown of Toronto. They share a cab, only for Margot to be mortified to learn that Daniel literally lives right down the street from her. A serendipitous encounter in its own way, for sure, but Margot knows she's already got a good thing going with her husband, Lou (Seth Rogen), a cookbook author specializing in all things chicken. Lou and Margot have been married for five years and have probably been with each other for twice that amount. The life they share is comfortable, their interactions lived-in and adorably quirky, and their sex life terminally uninvolving. What Margot sees in Daniel is that spark of a new carnal excitement. They begin to flirt, but she doesn't dare act upon anything with such a nice husband waiting at home. The decisions she makes from this point on will irrevocably change all their lives, and not even she, a few months down the line, will be able to say for sure that they were the right ones.

Newborn love and lust can be good for a thrill, but the latter element of one's relationship is misleading and superficial in the long run, destined to subside after a while to reveal the authentic person underneath. It's this bond, this compatibility, which decides whether a couple can see themselves growing old together. If they can't, they're destined for a break-up. The quandary Margot faces is that she can see herself with Lou for another half-century or so, and would probably be fairly content. Still, the thought plagues her that she's missing out, that Lou might not be the so-called "one." Out at a restaurant celebrating their anniversary, barely a word is exchanged between them. When she tries to get Lou to speak, he questions what they could talk about since they already know everything about each other. The key, some say, to a rock-solid relationship is when two people don't feel the need to speak to each other during a meal because being together is enough. There is a certain truth in this notion, but shouldn't Lou at least be trying? Margot knows he loves her, and she loves him, but is that enough?

Writer-director Sarah Polley (2007's "Away from Her") is a terrific actress in her own right, but her career behind the camera, though still in its relative infancy, might prove to be her destiny if she keeps making films as bold and unshakable and humane as "Take This Waltz." There is a hyper-realism to the world she creates right from the start, as Margot peeks silently into an ancient church only for it to be revealed that it's a Louisbourg fortress in the midst of putting on a reenactment. Sitting down along the nearby cliffs, she looks over to see a serene, picturesque lighthouse in the distance. And on her flight back, she is escorted by wheelchair to her respective gate. When Daniel questions her about this curiosity when he plainly saw her walking earlier without effort, Margot tells a flimsy lie, then comes clean: she's got a fear of connections, or being in between things. What if she were to go the wrong way and end up rotting in some forgotten terminal, she wonders. That Daniel—a virtual stranger at the start—would turn out to be living across the street from Margot is just far-fetched enough to be eerily plausible. When he continues popping up around town, even showing up on the bleachers as Margot and her sister-in-law Geraldine (Sarah Silverman) take an underwater aerobics class at the gym, the suspicion is that he might be a "what-if?" figment of her imagination. But no. He's there. Other people see him. Margot no doubt wishes they didn't. It would be easier that way.

In a film that never wanders away from Margot's point-of-view, Polley casts no judgment on her heroine. Margot comes to judge herself enough. She, like husband Lou, is an upstanding person, just trying to make her way. There isn't a malicious bone in her body, but as she continues to move toward Daniel's orbit, people are bound to get hurt. Michelle Williams (2011's "My Week with Marilyn") is almost stupefyingly brilliant as Margot, deeply empathetic and emotionally exposed as she faces the fate she makes for herself. Williams seems to get Oscar nominations with every other role she takes, but so far the award has evaded her grasp. This time, she deserves to win it, even if the year is only half over. To watch her onscreen is to catch a glimpse at a remarkable naturalism only the very best performers in history have, her every facial expression not only painting a picture, but writing a novel about who her character is and what she's going through. Without words to always speak and equipped with Polley's preference for impartiality, it is not always clear if Williams is content in the moment or breaking apart inside. This, too, simply lends itself more to the movie's open-ended allure; the use of subtlety is underrated.

For his part as Lou, Seth Rogen hasn't gotten to play such a layered figure since 2009's masterful satire "Observe & Report." Here, free of all eager-to-please comedic tics, he makes Lou lovable if gleefully unaware of his wife's feelings; he knows Margot through and through, but does he understand her all of the time? When the truth is finally revealed about an ongoing subplot involving a leak of cold water in the shower, the writing—and Rogen's understated delivery—is enough to break a person's heart. Luke Kirby (2003's "Shattered Glass") has a tougher time working his way into the viewer's positive graces, and doesn't ever pull it off. Maybe he's not supposed to. As Daniel, a rickshaw driver and part-time painter, Kirby must play "the other man," the potential home-breaker, and it doesn't help that most viewers will be able to see right through his magnetic charade. It's not that Daniel is a bad guy, either, but that he and Margot don't seem to have anything in common other than a sexual attraction that only heats up the more it is left unsatiated. Finally, Sarah Silverman (2011's "Peep World") is startlingly persuasive and affecting as Lou's older sister Geraldine, a recovering alcoholic who, in one of the picture's best scenes, sees right through Margot and stingingly speaks her mind about the mess she's foolishly created. It hurts Margot because she's knows she's likely right.

The sad fact is this: Margot is blinded by her untapped desires now, but if she acts upon then, there, too, will come a time when their honeymoon's over and she'll have to once more reevaluate Daniel's worth. That's the way romantic relationships go. Might Daniel be her soul mate? Or might she be giving up her soul mate—Lou—just for the chance to hop in bed with another for a fleeting fling? "Take This Waltz" sees no easy answers in Margot's actions, but it does see the tragedy in a certain corner of human nature where what we think is the right decision not always happens to be. Twice during the film, Margot rides The Scrambler, a beachside attraction that dizzyingly swings its passengers around to the classic tunes of yesteryear. The first time is with Daniel, and it's fun until she can't help her guilt from rearing its head. The second time is by herself, a provocative metaphor for her own mixed emotions. Never before has The Buggles' "Video Killed the Radio Star" been used to such hauntingly melancholic effect. Armed with lovely location photography in Ontario and Nova Scotia by Luc Montpellier (2010's "Cairo Time") and a soundtrack of quixotic indie melodies underscoring the daily ins and outs of Margot's present and would-be future, "Take This Waltz" is a vividly articulated piece of work difficult to break free from. No matter which path Margot chooses, it is the other one not taken which is destined to haunt her, and us.