Virtually no one would deny that adultery is a form of betrayal, but only rarely is it presented on screen as more than that. It's occasionally allowed to be sexy, usually muffled by guilt. What films almost never have the daring or sophistication to show is that adultery can be less a violation of life than a desperate, deeply urgent expression of it. We Don't Live Here Anymore, a wrenchingly intimate drama of marital infidelity, is the kind of movie for, and about, grown-ups that people used to talk about wanting to see but that just about no one makes anymore. Written by Larry Gross, who based it on two short stories by Andre Dubus (whose ''Killings'' became ''In the Bedroom''), and directed by John Curran, the film is at times harrowing to watch, yet it's also wry and delicate and absorbing. It's infused with the messy excitement of imperfect passion.
Jack (Mark Ruffalo) and Hank (Peter Krause) are 40ish English professors who are enjoying the summer off in their small, pastoral college town. The two jog and drink together, sharing weary macho confessions, yet they're only half honest; both are stuck in marriages that are quietly dying. As a drunken dinner party winds down, it takes us a few minutes to sort out, through all the flirting and dancing, who is actually married to whom.
Jack and his wife, the depressed, self-medicating Terry (Laura Dern), still need each other, yet the pressures of raising a family have ground them into a mutual unhappiness that they acknowledge in occasional screaming matches, only to sweep it back under the rug. Hank and Edith (Naomi Watts), by contrast, are the picture of smooth functionality, only it's a false picture: He worships their young daughter and devotes himself to writing fiction and poetry, yet he's also a poker-faced academic sleaze who has never been faithful.
Beaming out of this dark tangle of relationships is a ray of light: Jack and Edith are having an affair, and it's a blissful one -- erotic, emotional, a union of affection and spirit. As they meet up in town, then make love in the woods, Ruffalo and Watts forge a tender and moving bond, even as we're lured into the thorny practical intrigue of the characters' attempts to arrange an afternoon tryst. Do their spouses know? It's part of the film's subtle emotional force to recognize that this extramarital connection, while far from the vulgar act of ''wife swapping,'' really involves all four people. It's a playing out, maybe even an exorcism, of their fragile private demons.
''We Don't Live Here Anymore'' doesn't have anything like a conventional story. It ebbs and flows on currents of hope and anger, lust and despair, and if that's a high-wire way to make a movie, Gross and Curran have crafted it with stunning skill. It's the acting, however, that leaves one breathless. I won't soon forget Ruffalo's vulnerability, Krause's acerbic self-delusion, Watts' radiant sadness, or the flat-out brilliance of Laura Dern, whose face, twisted with love and torment, becomes an emblem of the movie's cathartic honesty.