Todd Field: I know what they did last summer

The director of In the Bedroom is obsessed with the evil that parents can do. Has he let fiction spill into real life?

"I remember," announces Todd Field, leaning forwards across the coffee table, "having a very, very vivid nightmare, where I killed someone. It was someone I knew, who I didn't like at all. And I may have thought once or twice: I really just want this person to disappear. And I woke up hysterical. I was so sickened by it that I knew at that moment I could never kill someone."

It's just what any prudent psychopath would say. As In the Bedroom – Field's first feature as director – demonstrates, it's impossible to distinguish the murderers from the rest of us. Until, that is, they pull the trigger, depress the detonation plunger, or shake the sachet of arsenic into the flute of buck's fizz.

You might know Field as an actor, a sparky supporting player in Hollywood product such as The Haunting and Twister; Anne Heche's fiancé in Walking and Talking; the blindfolded pianist in Stanley Kubrick's Gothic melodrama Eyes Wide Shut. Now In the Bedroom is accruing awards and nominations like a magnet dipped in iron filings, you may see a lot less of him: behind the camera is clearly where he belongs.

Based on "Killings", a short story by the late Andre Dubus, the film tells how a fatal shooting incident in a small town in coastal Maine fails to tear the local community apart. Sissy Spacek and Tom Wilkinson play Ruth and Matt Fowler, a married teacher and doctor, whose existence comprises choir practices, fishing jaunts and alfresco cook-ins. When Frank, their teenage son, (Nick Stahl), begins a relationship with a single mother, Natalie, (Marisa Tomei), the couple become uneasy – not least because Ruth suspects Matt would like an affair with her himself. The husband from whom Natalie is separated, however, harbours a more lethal kind of envy.

Field was under no obligation to consult Dubus, who long ago signed away the rights to "Killings", but the director coveted his approval. "It was self-serving," Field concedes. "I wanted him to tell me how great the script was." So he drove to Dubus's home in Haverhill, Massachusetts, where he found the author's son fixing up a deck in front of the house. For the last 14 years of Dubus's life, his sons did tasks such as this for him around the home. In 1986, the writer jumped from his car to aid a stranded motorist and, as he crossed the road, was struck by an oncoming vehicle. The accident cost him the use of his legs, and gave him the subject of a book of essays, Broken Vessels (1991).

"It was like an iceberg that had to melt," Field says, recounting their first meeting, and – as is his habit – doing all the voices. (Field's Dubus is a creature of cantankerous barks; his Todd Field a breathy, wide-eyed acolyte.)"'Why do you wanna make a movie of this?' He was real grumbly, not so happy about it. And his son took me out and said, 'Stick with him.' So we went back to the house and started talking about the story and he said, 'Why do you wanna make this into a movie?' I said, 'It's about that wife of his. That's what it's really about. In the story, she's deeply out of focus. Matt's always talking about her as if she's some little frail creature back home, but she's not.' And he said, 'What is she?' And I said, 'The way I'm thinking about it, it's the Scottish play. She's Lady Macbeth.' 'Yes! That's exactly why I love that story! You've got it! That's what it is! What else?'

"Once we got talking about it, that was it. We talked daily. As his son Jeff said, we had a short affair." Andre Dubus died in 1999; Field now has plans to film House of Sand and Fog, a novel by Jeff's brother, Andre Dubus III.

Perversely, Field's fidelity to his late mentor has produced a significant flaw in his film. The central events of "Killings" form the last couple of reels of In the Bedroom and those events – down to the nuances of the dialogue – are transferred exactly from page to screen. Let's issue a narratological health warning at this point: if you don't know the Dubus story – which, frankly, blabs its ending in the titular plural – you're about to hear how it winds up. It's the elision of the Fowlers and the Macbeths that's at the root of the trouble. Field's film brings us in so close to his protagonists, teases such eloquent performances from Spacek and Wilkinson, that it affronts us that they could get involved in something so vulgar as premeditated murder.

For Field, this is the whole point: "You think you've got to know this man, and you don't want to believe that about him. He doesn't seem capable of it." Sitting in the cinema, however, the moment Matt resorts to violence feels like betrayal – the substitution of something organic with the kind of narrative prosthesis found at the extremity of most Hollywood thrillers. "Most people are intensely unsatisfied by the end of the film, and worry about these characters," says Field. "There's an audible reaction when he raises the gun."

If Field had been cold enough to betray his mentor, the act of disloyalty might have made a masterpiece. When you see the film, you'll understand why. As it stands, In the Bedroom is still remarkable, unmissable, fully deserving the barrage of gongs and statuettes it will soon receive. But it seems like a great film with the wrong ending.

One of Field's changes to Dubus's story, however, may explain why he seems so absorbed by its murderous conclusion. "Killings" is set on the Massachusetts coast where Dubus made his home. In the Bedroom relocates these events to its director's native Maine, for reasons that might cause Field's family to choke on their lobsters.

"My parents were kind of similar to the Fowlers. There were things that happened between them that are intensely private that I will never know of. There was this public persona of being really good people. Such wonderful people. And I would have bought that, but there was always something about them that..." He trails off, lapsing into silence. "What if they weren't so... What if they were capable of doing something..." I'm now teetering on the edge of the thickly upholstered sofa in his Dorchester suite, fully expecting to hear how he found Ma and Pa Field tossing their blood-stained gardening gloves on the bonfire.

Clearly enjoying himself, he reels me in. "I remember I had all my wisdom teeth out, and I was home to see my parents. I remember hearing my dad on the phone. My dad's retired." Field begins to impersonate his father, holding his fist to his ear like a telephone receiver, his voice becoming clipped and low, his face grave and businesslike. "Yeah. Hmm. Right. Aha. We're just going to have to take care of it, aren't we? Um-hum." And he acts out a little family drama. Mr Field's voice is an impatient bass; his son's an incredulous whisper.

Todd: "Who are you talking to?"

Mr Field: "Oh. Nobody."

Todd: "Dad, who were you talking to?"

Mr Field: "Kirby."

"Now Kirby's a policeman. My dad trades guns. Sporting guns, just like the people in the film. They're hunters. But I swear to God, I couldn't stop thinking about that incident for weeks. I never got to the bottom of that. My parents still haven't seen the film. And if I brought it up now my dad would just say, 'I don't know what you're talking about.' But I know what I heard. That was nine years ago and I still can't forget it. It put a chill down my spine." Three weeks later, he asserts, he read "Killings".

Is he teasing me? Or helping us both out – "Meet the Director Who Suspects His Dad of Murder". You'd read the article, wouldn't you? You might even go to the film. But what will his parents think if they see these ruminations in print? "I'm safe because I'm overseas," he grins. "I can say all sorts of things because my parents don't have the internet. We're okay." We both giggle. I'm not really sure why. He's very smart, very charming. And almost believable.

'In the Bedroom' opens on 25 Jan