The makers of this adaptation of Tracy Letts’s award-winning play made a serious mistake in casting Sam Shepard as Beverly Weston, the Oklahoma professor and poet whose death brings his family together for his funeral—and a release of long-simmering hostility and long-held secrets. Not that Shepard is bad; indeed, he gives a fine performance as a tired man who, in all likelihood, takes his own life. It’s rather that his presence inevitably invites comparisons with his own plays about troubled families—works that are far richer and more compelling than “August: Osage County.”

Of course, screen adaptations of Shepard’s plays have by and large been disappointing, and wisely some of the best of them, like “Buried Child,” remain unfilmed. (The rare exception was the 1984 American Playhouse taping of “True West” with John Malkovich and Gary Sinise, which was exceptionally fine.) That’s because they’re extremely theatrical pieces that suffer when brought close-up; it’s the rare stage drama of this sort that can withstand camera’s glare. Once again, there is a major exception—“Who’s Afraid of Virginia Wolff?”

But that 1966 film had the benefit of Mike Nichols’ brilliant direction, and on the evidence of “August,” John Wells is no Mike Nichols. Nor, in this instance, does Meryl Streep outshine Elizabeth Taylor. That sounds like heresy, since Streep is by consensus a much greater actress than Taylor ever was. But the role of Violet, Beverly’s widow, is no match for Edward Albee’s Martha. It’s similarly showy—Violet is a nasty-tongued drug addict suffering from cancer—but lacks depth and resonance. And while Streep’s overwhelmingly histrionic turn certainly seizes on every sharp remark and outburst, it’s never able to invest the character with any real sense of humanity. One’s always aware of the effects she’s bringing to the part—and the fact that she’s playing to the rafters. That might work in the theatre, where there would be distance between the performer and the audience. But on the screen it comes across as simply too “big,” especially since Wells, perhaps understandably but nonetheless mistakenly, gives free rein to his star and enjoys showing her in exaggerated close-up. The result is that there seems to be entirely too much “acting,” in the old Jon Lovitz SNL sense, in Streep’s performance.

The other cast members fare somewhat better. “August” is basically a tale of mothers and daughters (in contrast to Shepard’s, which dwell on fathers and sons), and here Violet’s offspring are Barbara (Julia Roberts), Ivy (Julianne Nicholson) and Karen (Juliette Lewis). Of the three, Roberts comes off best as a businesslike woman in obvious fear that she might become like her mother. By contrast Nicholson is only fair as the mousy middle child, and Lewis is stuck with a part in its own way as exaggerated as Streep’s—the woman so man-hungry that she deludes herself about anyone in pants—and overplays it.

The men assigned to each of them are even less happy creations, and not especially well acted here. Barbara’s husband Bill is an owlish, recessive type, and Ewan McGregor seems utterly ill at ease in the part, while Dermot Mulroney is all too obviously sleazy as Steve Huberbrecht, the Florida wheeler-dealer whom Karen introduces as her fiancé. Ivy is the spinsterish type, but it turns out that she’s been seeing “Little Charlie,” played by tall, gangly Benedict Cumberbatch, who appears as uncomfortable in the role as McGregor is in his. Their hitherto secret relationship will be one of the causes of the last-act revelations obligatory in these sorts of family melodramas.

Cumberbatch’s Charlie is called little, in any event, not by reason of size but because he’s the son of Violet’s sister Mattie (Margo Martindale) and her husband Charlie (Chris Cooper)—who are probably the most subtly written characters on display here and the best acted by far, with Martindale and Cooper bringing some welcome depth not only to the dinner-table scenes but to one toward the close in which timorous Charlie finally gets up the courage to confront his abrasive wife about her belittling of their Little Charlie. Had the rest of the film been on that level, it would have been easier to commend. The major cast is rounded off by Abigail Breslin, as Barbara and Bill’s rebellious daughter Jean, and Misty Upham as the Native American Johnna, whom Beverly hires as housekeeper before going off to die, and who observes the Weston family circus with a calm detachment that can easily translate into compassion. Both are fine but hardly central to Letts’s high drama.

Wells has, in the usual fashion, attempted to “open up” the play somewhat by shooting in Oklahoma and including some outdoor shots. But most of the action still takes place in the family homestead, and the behind-the-scenes crew (production designer David Gropman, art director Karen Gropman, set designer Uldarico Sarmiento and set decorator Nancy Haigh) have done a good job in giving it a realistic look, just as Cindy Evans has done in terms of the costumes. Cinematographer Adriano Goldman captures everything in widescreen images that don’t prettify the locale, though he might have cut down a bit on the close-ups; and Gustavo Santaolalla contributes and understated score.

There’s a lot of talent and craftsmanship on display in “August: Osage County.” But ultimately it comes across as yet another adaptation of a well-regarded stage piece that hasn’t translated particularly well to the screen.