We don't know these people who inhabit Sarah Polley's "Take This Waltz" and that's the best thing about it: No one's a cliche; no one speaks dialogue the viewer could have muttered a beat or two ahead of the movie; no one hews to a mode of behavior fabricated to explain away his or her irrational behavior. Life is irrational, and so is Margot (Michelle Williams), the young married Canadian who, while en route home to bohemian Toronto, meets a kindred spirit named Daniel (Luke Kirby), and can't help letting her tidy life go to pieces.
Polley, the actress-turned-director whose "Away From Her" received so many honors in 2007, wades into extremely delicate territory with "Take This Waltz" (a song which, apropos of the director's staunch Canadianism, is by her countryman Leonard Cohen). Margot isn't fiercely unhappy -- she and husband Lou (comic Seth Rogen, pushing the acting envelope) have a seemingly untroubled relationship and are part of a tight-knit family where the only evident problem is Lou's sister Geraldine (Sarah Silverman), whose commitment to sobriety becomes as unsteady as Margot's commitment to Lou.
Margot and Lou make each other laugh -- something movie couples never do if they're headed for the shoals. Polley knows this, just as she knows that the dictates of the heart are simply inscrutable, and that the dissolution of a relationship isn't always accompanied by a bang. Sometimes, yes, it's only a whimper.
The weakness in "Take This Waltz," if that's what it is, comes courtesy of Luke Kirby, who can't quite make Daniel the irresistible attraction Margot thinks he is. But that's not necessarily bad -- unlikely lovers are hardly an anomaly in the history of fractured romance. Where Polley does go awry is in giving her characters' relationship too much time to simmer: Spontaneity may not always be the essential ingredient in romance. But it is in the movies.
PLOT Two young adults fall in love, and her marriage goes south. RATING R (language, sexual content and graphic nudity)
PLAYING AT Sag Harbor Cinema
BOTTOM LINE Consistently surprising, honest and beautifully acted by Williams