It's something of a critical cliche to say, "This is a film that rewards the patient viewer." But nothing less than the cliche will do to summarize "Take This Waltz." Which is ironic, since Sarah Polley's sophomore film is refreshingly free of cliches.

It's all the more an accomplishment, given her film's subject matter. Adultery has been a topic of storytelling almost since the birth of the form. But "Take This Waltz," in which Michelle Williams and Seth Rogen are the married couple and Luke Kirby the interloper, isn't a drama of grand romance or epic tragedy. It's something rarer: an authentic look at human relationships, and what we expect from them and ourselves.

Because it chooses realism, "Take This Waltz" is a slow starter. But the film is measured, not ponderous. And its third act is so unexpected, yet so undeniable, that you'll be glad you waited for it to turn into something more.

On screen
'Take This Waltz'
3 out of 4 stars
Stars: Michelle Williams, Seth Rogen, Luke Kirby
Director: Sarah Polley
Rated: R for language, some strong sexual content and graphic nudity
Running time: 116 minutes

Margot (Williams) and Lou (Rogen) seem happy enough after five years of marriage. Small irritations are present, of course. But so are big displays of love.

That's why Margot is so confused by her attraction to Daniel (Kirby), whom she meets quite by chance. It would have been simply one of those airport flirtations had they not gotten out of a cab and discovered they're practically neighbors. Margot is a good wife who can resist temptation -- but not when temptation is the man down the street.

The dialogue in this movie will strike many as strange. Margot and Lou correspond in a species of baby talk, while Margot and Daniel are more likely to insult one another. But that's how many old -- and potential -- couples indirectly communicate. When Margot finally tries to really talk to her husband, he's perplexed.

Sarah Polley started her career as an actress, and it's serving her well as a director. Williams might have been the best choice for a film like this, where so much of the "action" goes on inside the heroine's head and heart. There's a little girl quality about her that's showcased to great effect here, as a woman on the verge of 30 finally, awfully, grows up.

The verisimilitude extends past the dialogue. After a water aerobics class, a group of women shower together naked, young and old. Polley, who also wrote and directed the great 2006 film "Away from Her," can make magic and then show how reality inevitably intrudes on it. One of the best scenes of this melancholic movie takes place at an amusement park, as Margot and Daniel transfer their rush for each other into the adrenaline of a spinning ride. Suddenly, the music stops, the lights come back on, and a corpulent worker unlocks them from the car. If only they -- and we -- could always remember that the spell always comes to an end.