Ang Lee’s most famous films bear such scant resemblance to one another that it’s hard to believe they were made by the same man. How can the Asian auteur of Crouching Tiger, Hidden Dragon make the toxic chump Hulk? Is this really the same director who teased seven Oscar nominations from the frigid sheets of Sense and Sensibility? And what on earth is he doing with gay cowboys in his latest film, Brokeback Mountain? I have a theory about Lee — I think he was cloned at the National Taiwan College of Arts in 1975 to baffle American critics and bemuse Chinese fans.

The 51-year-old version perched on my sofa at the Dorchester in London is so softly spoken that you wonder how he copes with the ghastly egos of his glittering casts. Lee happily admits that he has little appetite for shouting, and even less for confrontation. “As the pictures get bigger I have to hurt people to get things done, even though it is against my nature,” he says, thoughtfully. It’s probably against the laws of physics, too. In the flesh, he doesn ’t look capable of denting a hard-boiled egg, let alone chinning a testy star.

Ninety per cent of Lee’s priceless talent is a steely obsession with tiny details. His choices might be deeply unpredictable, but the stitching is always immaculate. That said, even his most ardent admirers had to swallow hard when Lee arrived in Venice last September with a gay western starring the Hollywood poster boys Heath Ledger and Jake Gyllenhaal. As many of the critics sharpened their nibs, Lee scooped the Golden Lion, despite the stiffest competition in years. He repeated the trick just last weekend, taking best director and best film from the Los Angeles Film Critics Association.

For an Asian director who learnt to speak English only when his parents moved to the United States in 1978, Lee has an uncanny knack of finding truffles in stony literary ground. Annie Proulx’s short story Brokeback Mountain was first published in The New Yorker in 1997. It haunted Lee for years. Set in Marlboro country, it tells the story of two young ranch hands who spend the summer of 1963 eating beans and guarding sheep in frigid isolation. A cold night sparks a moment of intense, belt-loosening intimacy. The guilty drama is how the two loners fail to cope with their taboo feelings when summer ends and they are forced to part like strangers — the denial is heartbreaking.

Against all the odds, Lee has carved an epic love story out of the gay Wild West. He is unfazed by the way his film has been mercilessly pigeonholed. “It’s a subject that has never been treated seriously before,” he says. “What makes it fresh is not the story, it’s the combination of ingredients: the period, the people and the language. I consciously avoided the genre clichés. When people think of westerns they imagine gunslingers at noon. This is true rural life in the American West, with real human beings and real weather.”

Every grainy detail squeaks with authenticity. The amount of research makes the eyes water. Lee discovered that there are different types of bars for different cowboys, and a hierarchy of ranch hands who regard sheep herders as the lowest of the low.

What makes the film sing is Lee’s perspective as an “outsider”. He is fascinated with the way Americans project themselves. “As a foreigner you are less distracted by details,” muses Lee. “It makes it easier to see the big picture.”

Selling it, though, is another matter. His spectacular American Civil War movie about lawless Southern bushwhackers (mercenaries), Ride With the Devil (1999), was mauled by the critics. Lee felt hurt that he was accused of trespassing on an area to which he had no ethnic or cultural claim. But the staggering success of Crouching Tiger, Hidden Dragon, which was nominated for ten Academy Awards, eased the pain.

Lee’s magical ability to mint terrific, and radically different, romances in Chinese and English has given him enviable power in Hollywood. But he is wary of the privilege. He has little faith in politics, and you don’t have to dig deep to understand why.

“My father’s family were liquidated during the Cultural Revolution in China because they were landowners,” says Lee. “He was the only one to escape. I was born and brought up in Taiwan. But you absorb the trauma. My parents had no sense of security. It was as if the world could turn against them at any moment.”

Lee’s greatest personal battle has been trying to reconcile his father to his career. The paternal lead in his great Chinese trilogy, Father Knows Best, was based entirely on him. “I was afraid of what he would say, but he never uttered a word. After Sense and Sensibility he said to me: ‘Great. You’ve made some movies. You can retire at 50 and do something sensible, like teaching.’ He softened after Crouching Tiger. Not because I made money, or won an Oscar, but because I hadn’t messed up my family life.

“The first time he encouraged me was when I told him I was going to give it all up after Hulk. I was ragged. Every cell in my body was screaming for rest. He held my hand and said: ‘Please don’t. You’ll be depressed.’ Then he said he loved Hulk. Two weeks later he passed away. That’s when I decided to make Brokeback Mountain.”

Lee pauses, then smiles. “I never did tell him it was a gay movie . . .”

Brokeback Mountain opens in London on December 30, then nationwide from January 6