If British theater has its own tradition of "kitchen-sink realism" -- quiet, dreary movies about working-class life -- American film has begun to specialize in what might be called Cheerios realism. These movies offer the kinds of challenges actors relish, depicting the lower fringes of middle-class existence on disordered sets laden with cereal boxes, bottles of Mrs. Butterworth's syrup, jars of French's mustard and so forth. This genre often intersects with the separate but related genre of stoner realism, in which characters fire up a doobie at every opportunity. "You Can Count on Me" certainly ranks highly on both the Grape Nuts and the ganja scales, although it may be difficult for any movie to top "Wonder Boys" in either category. (See also "Erin Brockovich," "American Beauty" and "The Slums of Beverly Hills," among others.)

Lonergan's direction in "You Can Count on Me" is clean and unshowy, and lets the characters drive the movie. He also tells a story in which things are not quite what they seem, and not in some dumb-ass thriller sense, either. His focal point, at least at first, is Sammy (Laura Linney), a single mom of 35 or so in Scottsville, an unassuming town in upstate New York. (Habitués of the Catskills region will recognize the picturesque main street of Margaretville, N.Y., as a principal location.) Scottsville is the kind of place where a local minister, played by Lonergan, won't even offer an unqualified opinion about adultery. "Well, it's a sin," he says. "But we try not to focus on that right off the bat."

Sammy's a slightly prissy, WASPy blond with a Lands' End wardrobe who works as a loan officer in a local bank. After sex with her on-again, off-again boyfriend Bob (Jon Tenney), she tells him, "Thanks for a lovely evening." She's not cracking a joke, or being a coquette. She's just being a nice PTA mom who is politely melting into middle age amid the pretty but unremarkable Scottsville scenery.

When her deadbeat, pothead brother Terry (Mark Ruffalo) turns up in town, Sammy is thrilled to see him. We, on the other hand, are less certain how to feel about this development. Before we even know who Terry is, we watch him in a crappy apartment with a sideways stereo speaker for a coffee table, telling an unidentified woman, "I am not the kind of guy that everyone says I am." People who feel the need to say that are generally wrong. Since Terry gets madly stoned in the bathroom of a moving bus, and asks Sammy for money almost as soon as he sees her, we feel justified in suspecting that he's exactly that kind of guy.

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