Can Dallas Buyers Club win Matthew McConaughey an Oscar?

McConaughey plays Rust Cohle, an intense, troubled cop, who’s lost his wife and daughter by working too deep under cover, and is now given to depressing philosophical monologues on “the penance that life is”.

“I wouldn’t wish anyone else to be in his head. He’s a real island unto himself, but I found that really interesting,” enthuses McConaughey. “I felt like I knew who this guy was. I loved this guy’s mind. I fell in love with the words coming out of his mouth.”

More sophisticated by far than most usual cops-and-killers offerings, the show also stars McConaughey’s real-life buddy, Woody Harrelson, as his mismatched investigating partner, Martin Hart, in the complex story of the hunt for a serial killer, which spans a period of 17 years.

On a bright winter’s morning, I meet 44-year-old McConaughey at a hotel in Beverly Hills to discuss the tense, dark drama, the US premiere of which last month gave HBO its biggest audience since the launch of Boardwalk Empire in 2010.

The actor’s extreme weight loss – shedding 48lb (almost three and a half stone/22kg) – to play Ron Woodroof, a homophobic electrician who contracts HIV, is well documented. Paparazzi shots of the former amateur triathlete looking frighteningly emaciated prompted widespread concern as to whether he had, perhaps, gone too far for the role.

Today, for the record, he still appears significantly slimmer and more angular than in the past, despite having regained all but 8lb of the original weight. But his career, having been through a no-less-dramatic transformation, is now in substantially different shape, too.

McConaughey is no longer viewed as the tousled hunk of How to Lose a Guy in 10 Days (2003), Failure to Launch (2006) and The Wedding Planner (2001), but over the past three years has demonstrated some serious acting muscle in a clutch of critical hits including The Paperboy, alongside Nicole Kidman, the black comedy Killer Joe, and Mud, in which he played the title role, a sensitive fugitive in Mississippi who befriends a pair of adolescent boys while trying to win back his childhood sweetheart.

Even in Magic Mike, the tale of a male strip troupe, McConaughey brought some impressive weight to Dallas, his thrusting, thong-wearing alter ego.

He is somewhat reticent, however, to discuss in too much detail his professional about-turn. “I haven’t really been looking in the rearview mirror for a while,” he shrugs. “There’s not a true answer to fit the narrative of a then-and-now scenario, a coming-to-Jesus moment, because that didn’t happen. This is an evolution in my career, as I’ve evolved in my own life.” However, he will admit that after a long spell of starring in rather fluffy fare, he decided to “put the brakes on. I said, ‘I’m not going to rush into work. Let the work find me.’ ”

Like Mud, True Detective is set in the South, this time in the bleak backwaters of rural Louisiana – among ugly refineries and ramshackle trailer-park communities – which neighbours McConaughey’s native Texas, where he grew up the son of a kindergarten teacher, Kay, and a gas-station owner, “Big Jim”.

“There’s just a lot of great stories that come out of the South. Time runs at a different pace,” says McConaughey who, in person is unexpectedly intense and loquacious, radiating a fearsome focus; it’s not tricky to see why he associated with Cohle, or how seriously he took his recent weight-loss regime.

“It honestly was not that difficult,” he claims. “It became a fun challenge. When people would say there was no way I could get down to that weight, I knew I could.” Six feet tall, he dropped from 183lb (13 stone 1lb) to 135lb (9 stone 9lb). The first month, he admits, was tricky. “You have to reprogramme all of your habits, and the days get so long. You think it must be lunchtime already, and it’s only 9.30 in the morning,” he says. “I kept a diary of my nutrition the whole way through, and it is something I will probably share at some point because it was quite the adventure.”

The McConaughey Diet involved studiously avoiding temptation: “I didn’t go out to restaurants and say, in a steak house, that I’d have 5oz of fish, no oil. I wasn’t going to put myself through that.” And, a sun-worshipper by nature, he became a veritable hermit, holing up at home in Austin, Texas. “This guy needed to be pale, so I didn’t go outside in the sun for six months. In the summer, I gave myself a winter.” He became weak: “I would do five push-ups and be sore. I would run 30 feet and my legs would lock up.”

He claims to have found the experience beneficial mentally and psychologically. “I had to relearn how to entertain myself, because I wasn’t going outside, I wasn’t going to dinner, I wasn’t going to social places. I was writing so much more, reading so much more. It ended up being this really fun adventure, internally,” he insists.

But there were some severe side effects. “As soon as I hit 143lb [10 stone 3lb], I started losing my eyesight,” he tells me, almost casually. He looked into the condition and found the same had happened to the IRA hunger strikers in Northern Ireland 20 years ago.

It was undoubtedly the most brutal regime he’s undertaken for a role, but far from the first; for Magic Mike, he locked himself away from the world, working out and eating ultra-carefully “to get in wicked-looking shape”, but for the post-apocalyptic Reign of Fire in 2001, he took an infinitely more fun-sounding method approach. ‘I went out to my ranch in Texas by myself for two months, and would wake up every morning, have a shot of tequila, then spend the day wrestling cows and picking up 20lb boulders.”

With his evident penchant for extreme physical prep, it’s something of a relief to learn that his wife, the Brazilian model Camila Alves, keeps a close eye on him. “I am very lucky that she agreed – in fact, she suggested – that when I go to work, we all go together,” he beams. Along with Alves, their three young children – Levi, five, Vida, four, and two-year-old Livingston – travel wherever their father is filming. They all relocated to Louisiana for True Detective with him, too, where the gruelling six-month filming schedule regularly involved 16-hour days and where, according to his co-star Harrelson, McConaughey stayed in character as Rust Cohle for most of the day.

“I was a little scared of how it would work at the beginning,” McConaughey admits of having his family so close to the set. “But it has actually helped,” he nods. “The more secure a man is at home, the higher and wider he can fly outside of it.”

Actors' crash diets:

Tom Hanks lost 26lb for Philadelphia (1993) and won an Oscar. He went on to lose 50lbs for Castaway.

Christian Bale starved for four months before The Machinist (2006), losing 62lbs.

Michael Sheen lost two and a half stone on the cabbage diet to become Kenneth Williams for Fantabulosa! (2006)

Matt Damon lost 40lbs for Courage under Fire (1996). He needed medical supervision for months afterwards.

See True Detective, Saturday 9:00pm, Sky Atlantic

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