When Tracy Letts accepted the Tony for best play in 2008, he finished his speech with a bitter kicker. "They did an amazing thing," he said, in reference to the backers of August: Osage County. "They decided to produce an American play, on Broadway, with theatre actors."

Those theatre actors who made Letts's mammoth play a sensation in Chicago, on Broadway and then in London have now been swapped for movie stars in the film version. The producers this time round, including George Clooney and Harvey Weinstein, have also done an amazing thing: produced a foul-mouthed and dark movie for grownups, maintaining the bulk of Letts's riotously funny but brutal three-hour play. The film has had varying degrees of Oscar buzz since the first trailer debuted – horse-race chatter that irritates Letts.

"They take all the good, serious adult movies," he says, "and cram them at the end of the year and see which one of them is going to make it into the Kentucky Derby." But if August: Osage County runs well, it won't be Letts and his wisecracks taking the stage this time. "It's not a Tracy Letts play, it's a John Wells film," he has said. Speaking to me in Manhattan in November, the writer and actor, who starred in the last season of Homeland as power-hungry Senator Andrew Lockhart, measures his words carefully: "It's the movie biz. It involves a lot of collaboration. And it's a medium in which I'm not the boss."

Intentionally or not, Letts is echoing the titanic last line of the Pulitzer prize-winning play's second act, in which eldest daughter Barbara (played in the film by Julia Roberts) wrestles her mother Violet (Meryl Streep) to the ground, pulls a bottle of pills from her hands and bellows: "I'm running things now!"

It's just one of the many emotional climaxes in the play, which reunites an Oklahoma family, the Westons, in the rambling home of Violet and her husband, Beverly, a poet professor who has suddenly gone missing. Violet is suffering from mouth cancer and constantly self-medicating, and Barbara is facing the end of her marriage – yet they may be the least-suffering members of the Weston clan. There's incest, attempted rape, familial abuse and horrible home cooking to deal with, not to mention the general despair of living in the US in 2007. The Bush-era resonance of the play is gone – Letts insists it wasn't that important to begin with – and the running time has been cut to a manageable two hours. But August: Osage County remains unscarred onscreen.

This isn't the first time Letts has adapted one of his own plays for the cinema – director William Friedkin turned both Bug and Killer Joe into low-budget, critically acclaimed but little-seen indies. Letts wasn't on the set for either production but has described working with Friedkin as a "continual collaboration", with "fewer cooks in the kitchen" than on something such as August, which was stage-managed by the the Weinsteins back in 2008.

Letts met with several potential directors before the studio settled on former ER executive producer Wells. "I don't really know what that was about since they wound up hiring John without me having that meeting," he says. "I don't know what process they were going through to arrive at that decision." As the cast was assembled – it includes Chris Cooper, Juliette Lewis and Benedict Cumberbatch among others – the cooks in the kitchen got more and more opinionated; Letts describes them as "people who know a lot about making movies, and they all have a lot of opinions on the way it should be done". He wrote each draft of the screenplay and fought certain changes to the very end, but is happy to acknowledge his backseat role: "I know going in that I'm not going to be the boss. I'm not going to be making the final decisions." Is there some relief in giving up that control? "Sure. In sitting and going: 'Hey, if you fuck this up, it's on you, not on me.'"

After the film's premiere at the Toronto film festival in September, Letts and Wells admitted in an interview with the Los Angeles Times that they were still considering changing the film's ending, which tacks on an extra scene slightly sunnier than the play's pitch-black finish. Letts concedes that he was fighting for changes in Toronto – "Material that I felt still had a place in the work, some things I felt were screwy about the ending" – but insists that talk about a potential new ending was overblown. "August: Osage County has always only ended one way. However, when you're ending a film, there are a lot of dials you're fiddling with. How long does the last shot go on for? Is that the last shot? When do the credits start? When does the music start? Is that the music? These are the things we were trying to figure out, and we didn't like the tone at the end of the movie and we hadn't gotten it right. Nobody was ever arguing to give us a smiley ending. It would be disastrous if they did."

The biggest changes in August's translation from page to screen come from Osage County itself, presented as bleached cornfields and endless horizons broken up by black highways. Driving alongside her estranged husband early in the film, Barbara wonders aloud who decided to settle in this "flat, hot nothing". Later in the film, Barbara is driving with her mother down a similarly bleak highway when Violet suddenly escapes, running off into a hayfield with Barbara in hot pursuit. Letts identifies that scene as one of his favourites. "It got at something thematically that I always thought was important. For somebody who's grown up there in Oklahoma, I felt – and John agreed, he's also from the plains outside of Denver – that there was a kind of claustrophobia inherent in growing up in that part of the country. It's an odd kind of claustrophobia in that you can see 50 miles to the horizon."

Letts named the characters at the centre of his play the Westons, putting the tantalising American promise – "Go west, young man" – right in the middle of their names, then trapping them inside a crumbling house and watching them tear each other apart. The film makes the west visible in the story, but just a larger trap – at the end of that scene in the hayfield Barbara tells Violet: "There's nowhere to go." Letts explains: "I thought it was important thematically to August: Osage County that they were representative not only of Oklahoma and the plains, but even the larger picture than that. That they were representativeof the country on some level." He pauses. "Yeah, the Westons. I always liked what that word evoked."Though Letts shakes off the suggestion that his own Oklahoma roots continue to fascinate him, he's returning to the west with his next screen project, an adaptation of John Steinbeck's The Grapes of Wrath. There's already one famous film version of it from 1940, starring Henry Fonda and directed by John Ford, but Letts is unfazed. "There's a heck of a lot in that book that they couldn't do in that movie, they couldn't approach doing. Believe me, the book and the movie are really important to me and the people where I come from. But it deserves revisitation."

Letts won a second Tony in June, the best actor prize for his role as George in the Broadway revival of Who's Afraid of Virginia Woolf? His acceptance speech from that night is an enormous contrast to the wry Osage County one – he seems genuinely overwhelmed and emotional. "It was a nice acknowledgement of an exhausting couple of years of work." Does that mean writing August: Osage County is no longer the hardest thing he's done, as he's suggested before? "There are different kinds of hard. There's digging postholes – that's hard. It's not that kind of hard. Virginia Woolf was certainly one of the hardest things I've ever done, but in a different way."

Letts was in the midst of Virginia Woolf rehearsals when the Osage County movie went into production, which kept him away from the set entirely. But he says that, even after years of adapting the film and years before that of writing the play and mounting the production, he wasn't done with the story until he saw the film's final cut, a week before we spoke. "I finally felt like I don't have to fight for anything."

So is he done with it now?

"Oh, I am so done with it."

August: Osage County goes on general release in the UK on 24 Jan