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Kenneth Lonergan was responsible for the script of “The Adventures of Rocky and Bullwinkle,” but one should be ready to forgive and forget that after seeing his beautifully crafted, surprisingly touching new drama about a strained sibling relationship in a small New York town. At first glance it might seem that with a few minor edits, “You Can Count on Me” could have been a Hallmark Hall of Fame TV special. But it would have been an exceptional one: the picture is so nicely gauged and incisively written that, despite a few flaws, it emerges as one of the small gems of the fall film season.

The picture centers on single mom Sammy Prescott (Laura Linney), a bank worker who lives on the old family homestead with her young son Rudy (Rory Culkin). Her staid, sedate existence is suddenly shaken by two simultaneous events: the arrival of a new boss, a rigid and unyielding manager named Brian (Matthew Broderick) who objects to her rearranging her hours to meet her maternal responsibilities, and the return of her ne’er-do-well brother Terry (Mark Ruffalo), whom she loves dearly but regards as hopelessly unreliable. Lonergan’s sharp, witty screenplay effortlessly reveals how the interaction among these characters changes each of them. Sammy is liberated by her brother’s example, eventually linking up with the married Brian, but is also unsettled by Terry’s influence on her son. Terry strives to play a settled, familial role, but his naturally rebellious temperament sporadically derails his efforts. Fatherless Rudy is inevitably drawn to Terry, but often is the more mature of the two, and is understandably afraid of being abandoned by his uncle as he was by his dad. And Brian is torn between love for his pregnant wife Mabel (V. Smith-Cameron), his attraction to Sammy, and his urge to run a tight ship.

What’s so extraordinary about “You Can Count on Me” is that it’s not only beautifully written and directed but wonderfully acted as well. Lonergan’s script captures the rhythms of the ever-changing relationships among the characters with almost startling sensitivity, and his helming exhibits a subtlety and lightness of touch almost absent from American movies nowadays. Linney and Ruffalo are absolutely outstanding as the squabbling but supportive siblings, succesfully showing their mutual love as well as the inevitable tensions that arise from their very different lives and perspectives; both offer splendidly shaded, convincing performances. Culkin is charming as the precocious yet solemn Rudy; along with brother Kieran (who was so fine in 1998’s underrated “The Mighty”), he proves that the acting talent in their family is widely shared. Broderick will come across to many as overly mannered (it’s almost as though Brian represents the later stage in the life of Jim McAllister, the hapless teacher the actor played in Alexander Payne’s brilliant 1999 “Election”), but he proves an excellent comic foil, and at points draws some poignancy from the character too. There are also good supporting turns scattered throughout the piece (Jon Tenney is nicely restrained as Sammy’s hometown beau), including one by Lonergan himself as a slightly befuddled but well-intentioned priest.

“You Can Count on Me” has a richness of texture and characterization that’s truly rare in contemporary films, even small, independent projects such as this one. It has a few minor flaws: there are far too many establishing shots of the Prescott home to indicate scene changes, for example, and especially at the beginning, Lesley Barber’s Bach-based score strikes the listener as overly precious. But overall the picture is so pleasurably quirky and off-kilter, and the performances so nuanced and apt, that it disarms criticism. By all means, seek out this jewel of a movie.