When fantasy becomes reality, it's bound to disappoint -- especially if it has anything to do with forbidden romance. An old plot device, but the sweet, slow-cooked version concocted by Canadian writer/director Sarah Polley in "Take This Waltz" has enough charm to coax us into watching one more, with occasional impatience.

Set in Toronto's boho, color-drenched Little Portugal neighborhood, the story centers on Margot (Michelle Williams), a wannabe writer married to successful cookbook author Lou (Seth Rogen). On a solo jaunt to a tourist attraction, Margot meets Daniel (Luke Kirby), a handsome artist who pays his rent as a rickshaw runner, and who, conveniently, turns out to live nearby.

Instant attraction, smoldering flirtation, unbearable anticipation, inevitable comparison of bird in hand to bird in bush, and agonizing guilt ensue. As waif-wife Margot, who tells Daniel she's "afraid of being afraid," Williams skitters as close to twee as she ever has, just one adorable vintage sundress or wobbly lower lip away from setting our teeth on edge. As always, though, she pulls it off, even during a didn't-see-that-coming sex-scene montage set to the Leonard Cohen song from which the film takes it name.

Two roles are played somewhat against type by a pair who have made their reps on being crassly hilarious -- Seth Rogen as Lou and Sarah Silverman as his sister. Rogen, the film's most likable character, runs away with the best tearjerker scene, a stilted monologue in which his thoughts run over each other as he processes a devastating announcement by Margot, the audience observing him from her point of view all the while. That alone should snag him all the serious roles he wants.

Silverman adds some welcome wisecracks, but isn't quite convincing during her more serious moments as an alcoholic in recovery. Kirby's Daniel also leaves something wanting, as he seems too cool a customer -- at times just this side of creepy -- to attract the curled-up ball of need and uncertainty that is Margot.

Polley has mastered the art of the dreamy scene, lulling her audience into floating along with whatever light- and sound-driven flight of fancy her onscreen protagonist is experiencing, as she proved in her 2006 Oscar-nominated adaptation of the Alice Munro story "Away From Her." But her pacing sometimes suffers, as she can't resist one more whirl of the carnival ride, one more lingering shot of Williams' alternately rapt and pained face. She does avoid making the ending either too predictable or too unbelievable.

"Life has a gap in it, it just does," Margot's sister-in-law tells her. "You don't go trying to fill it like a lunatic."

In "Take This Waltz," Polley doesn't fill the gap. She just makes life on either side of it both attractive and a drag.

Kristin Tillotson • 612-673-7046