If Tom Kalin's latest film was fictional, it might easily be dismissed as lurid melodrama. But Savage Grace is a true story of privilege, sex and murder that combines the overheated camp of soap opera with the toxic family tragedy of a Tennessee Williams play. Proof that truth is sometimes not just stranger than fiction but considerably darker too.

Based on a cult book by Natalie Robins and Steven Aronson, Savage Grace traces the dramatic rise and fall of Barbara Daly Baekeland, wife of Brooks Baekeland, dissolute heir to the Bakelite plastics fortune. Barbara, played superbly by Julianne Moore, was an unstable, suicidally depressed social climber with rapacious, transgressive sexual appetites.

The film concentrates on her incestuous relationship with her son Antony, played by Eddie Redmayne, who finally took grisly revenge on his mother in November 1972. “That oedipal bond between the two of them is what made the film attractive in the first place,” Kalin says. “It was sensational, shocking, true-life material, but it also had the resonance of Greek tragedy.”

The film's mother-and-son sex scenes have raised a few eyebrows. Especially their remarkable final coupling, brutally cold and remorselessly drawn out, which propels the story towards its bloody end. “I've had some quite intense reactions,” the 46-year-old director admits: “‘Why the hell are you putting it on screen for this long?' Because I think it's necessary dramatically, to get to that place where you see what's going on between the characters. The detachment of intimacy. He's all strung out, chain smoking, when she comes home. It's sort of lion taming. She calms him down by having sex with him.”

This scene is partly speculative, but grounded in ample evidence of an incestuous relationship. “Barbara certainly told people she was sleeping with Tony to cure him of his homosexuality,” Kalin says. “I find that way too facile an explanation.”

On one level Kalin intends us to empathise with Barbara, despite her monstrous and abusive narcissism. A failed movie star with a family history of suicide and depression, she married into money. “There were limited roles for women in that time,” Kalin says. “I joke somewhat when I say this, but Barbara would have been Madonna had she been born in a different time.”

Like Swoon, Kalin's first feature, Savage Grace takes a scalpel to the sadism and arrogance of America's upper classes. This is coincidence, he insists, although his own relatively humble background helps to explain his personal empathy with Barbara's social insecurity.

The youngest of 11 children, Kalin was born in Illinois. His Irish Catholic family had limited money but a high regard for culture and education. “I grew up hugely aware of class,” he says. “It's the elephant in the room of American society.” With six brothers, Kalin's family was a “relatively butch environment”. He is homosexual but he says that his family is supportive. “I've been involved with the same person for 15 years. He's a part of their life, so I've been really lucky.”

Kalin's elegantly subversive Swoon, released in 1992, revisited the notorious case of the gay lovers Nathan Leopold and Richard Loeb, who killed a child in 1924. It earned him a place in the so-called New Queer Cinema movement alongside Todd Haynes and Gregg Araki - directors whose work challenged coy gay stereotypes with knowingly ironic, often confrontational attitude.

Kalin came to film-making from the Aids activist groups Gran Fury and Act Up, where he met Haynes. But he argues that New Queer Cinema was largely a short-lived media invention, necessary at the time but not today. “Homosexuality in popular culture is not the shocking or rare thing it was in late Eighties and early Nineties,” he shrugs. “For good or bad, nowadays we have situation comedies where gays are ‘just like everybody else', ha! But I don't know if that's progress, that desexed, middle-class, collie-in-the-back-of-the-station-wagon representation.”

Savage Grace is released on July 11

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