Five years into their marriage, the freelance Toronto writers played by Michelle Williams and Seth Rogen in “Take This Waltz” have drifted, rudderless, into a harbor that is anything but safe.

They’re cute together, but the act has begun to curdle: The reflexive baby talk for laughs, the weirdly hostile banter (“I love you so much I’m gonna inject your face with a curious combination of swine flu and ebola”) and a troubling lack of easy intimacy all spell trouble.

In writer-director Sarah Polley’s follow-up to her first feature, the fine 2006 drama “Away From Her,” trouble carries a rickshaw. Across the street from Margot and Lou’s house, a fledgling artist and part-time rickshaw puller presents a tempting alternative to sweet, safe Lou.

I see why “Take This Waltz” has been described by some as exasperating. Some of it is; some of it is intentionally so. Margot’s itchy dissatisfactions aren’t always explained or justified. On the other hand, Polley’s film makes it all too clear that Lou and Margot haven’t the glue, the promise, to keep both halves of the marriage we see happy. It’s a measure of this challenging film’s success that Margot’s decisions lead only to more questions, rather than a single, neat rom-com answer.

Luke Kirby plays Daniel, the hunky bohemian with the bedroom eyes. For once, I suppose, the shoe’s on the other foot: Usually it’s a third-billed emblem of female temptation, not male, that serves this particular story function. Polley’s script engineers the initial meeting of Margot and Daniel while Margot’s on assignment in Nova Scotia, writing a pamphlet for the Canadian parks service. Once everyone’s back home in Toronto, Lou’s sister (Sarah Silverman in an excellent, needling but subtle portrayal of a wary recovering alcoholic) senses the attraction between Margot and the guy across the street long before Lou does.

“Take This Waltz” takes its title and mood of romantic fatalism from the Leonard Cohen song heard during a lovemaking montage of indeterminate reality/ fantasy status. Polley wonders the same thing in nearly every scene, no matter who’s in it. Is a comfortable marriage really marriage enough?

Margot, as played by the unerring and complicatedly honest Williams, might never be content. Showering together after an aquatic aerobics class, with bodies of all shapes and sizes and ages together, Margot and her sister-in-law trade guarded conversation in an unguarded setting. “Life,” says Silverman’s cynical Geraldine, pointedly, “has a gap in it. It just does. You don’t go crazy trying to fill it, like some lunatic.” But some do.

Polley’s a strong enough director to make one wish “Take This Waltz” had left more to purely visual passages, such as the dizzying fun-house ride between Margot and Daniel scored to “Video Killed the Radio Star” by The Buggles. This scene is neither joyous nor damning; rather, it’s about a dozen things at once. The film’s super-saturated color palette deliberately crowds the characters. Perhaps less deliberately, Polley’s characters exist in a realm of dreamy yearning, gliding along, as Chris Hewitt of the St. Paul Pioneer Press put it, apparently supported by “the sort of jobs you can afford only if you live in a place with nationalized health care.”

Is Margot a flake, or a woman simply following her heart, right behind her loins? Not all of “Take This Waltz” works, and some of the writing is very on-the-nose. “I’m afraid of connections,” Margot tells Daniel early on, regarding her fear of flying but speaking all too metaphorically. The movie doesn’t need that sort of underlining.

The actors are excellent. Rogen falls very comfortably into the role of a 29-year-old who has fallen very comfortably into a living thing — a marriage — and stopped working on it. Williams is the ringer, a welter of difficult, delicate feelings in search of a resting place.