"We shouldn't be here."

That's the sense you get watching August: Osage County -- that you're peering in on moments so intimate and painful that no one should witness them, perhaps not even those who are a part of it.

In fact, that's what many characters in the movie — an adaptation of Tracy Letts' Pulitzer Prize-winning play — decide for themselves. They don't want to be part of it, either. In this story of an uncomfortable family reunion, time is marked by cars pulling out of their dusty Oklahoma driveway at regular intervals, never to be seen again.

The impetus for the gathering is the disappearance of Beverly (Sam Shepard), patriarch, poet and alcoholic. His wife, Violet (Meryl Streep), suffering from oral cancer and perpetually high on a laundry list of sedatives and painkillers, calls on her sister and her three daughters, along with their various significant others, to come to her aid during the developing crisis. And with everyone under one roof, years of family dysfunction come to bloom at the end of her cruel, pill-addled tongue.

Letts reworked his own play for the screen, as he also has done in recent years for adaptations of his plays Bug and Killer Joe,and the dialogue — harsh, blackly comic, filled with exchanges of sulfuric intensity — is of a piece with those works.

Interestingly, the writer doesn't bother making these screen ventures hugely distinct from their stage inspirations; their staginess works, though, because his characters seem trapped within their own dire lives just as much as if they were locked within the limited space of a theater.

And theatricality makes a kind of sense, really; these are lives lived without any letup from the heightened drama. Every member of this cast is a walking, talking open wound.

Eldest daughter Barbara (Julia Roberts) is separated from her philandering husband Bill (Ewan MacGregor), and can't relate to her teenage daughter Jean (Abigail Breslin). Put-upon middle daughter Ivy (Julianne Nicholson) is conducting a secret relationship the family wouldn't approve of, desperately seeking to escape Oklahoma and her mother. Airheaded youngest daughter Karen (Juliette Lewis) is engaged to a Ferrari-driving, thrice-divorced sleazebag (Dermot Mulroney).

Everyone here has pain, everyone has secrets, and while we join these characters for a short time, it's easy to see that the cycles of lies, distrust, and abuse go back for generations, clinging to this family like the hot summer dust of the empty plains that surround them.

Director John Wells, a TV veteran directing his second feature, is at his best when he's invisible. When he steps back and lets this formidable cast act their hearts out, the movie is a deeply affecting — and disturbing — storm of complicated emotions.

Roberts is especially impressive — bitter, broken, relatively restrained until a rage and profanity-filled climax lets out a lifetime of resentment. Chris Cooper is absolutely essential as the audience surrogate, reaching the end of his rope after 38 years of involvement with the family; he delivers perhaps the best monologue in the movie, wondering why there's such a dearth of respect, kindness or basic decency in this house. But everyone here is uniformly exceptional, even if Streep could stand to dial back her swinging-for-the-fences performance just a notch.

Letts' drama is built to leave the viewer uneasy and dissatisfied; don't expect all or even most of those wounds to heal, either during the movie or ever in these characters' lifetimes. But despite the standout performances, the film itself leaves one wanting as well, in ways that Letts likely didn't intend.

When Wells doesn't remain invisible, he steps in mostly to blunt the impact of the nonstop trauma onscreen. Some might welcome a little salve, but it runs counter to the harsh, uncompromising honesty of Letts' work. We don't need intrusively sad music to clue us in about how sad these lives are. We don't need blandly pretty landscape shots to act as contrast to the ugliness elsewhere onscreen. Letts arms his characters with words as weapons, and William Friedkin's prior Letts adaptations let them retain their raw destructive power.

This film, a little too often, feels calculated to dull those verbal blades for awards-season palatability; when things get bad, Wells comes in again with a directorial "There, there, it'll be OK." But in Osage County, Okla., things are very much not OK.

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