‘I definitely have an alpha-male aspect to me,’ said Jake, who could pass for an athlete

The film is an original, thought-provoking and very funny exploration of love and sex, in which a relationship between Viagra salesman Jamie (Gyllenhaal) and Maggie (Anne Hathaway), an ostensibly confident woman with early-onset Parkinson’s, develops into love.

‘Jamie is the ultimate seducer and would have been perfectly happy to float through life minus the burden of responsibility or connecting to anyone – until he meets Maggie,’ explains Gyllenhaal.

‘There’s not a whole lot of actual sex in the film. There is a lot of us talking with our clothes off, beforehand or after, and I think that’s more real. You don’t have a sheet draped across Annie’s chest, because people don’t tend to do that, do you know what I mean?’ He grins.

‘It’s uncomfortable when you’re naked on set, but I do feel like an old hand at it at this point. I’ve done some pretty crazy things already.

‘My parents taught me to feel comfortable about my body. They told me there’s a beauty in whatever you are. Also I feel it’s very important to portray love and sex in the right way.’

He laughs. ‘Besides, in our case, we’d already had faux movie sex in Brokeback Mountain. So we were relatively comfortable.’

Gyllenhaal won’t be drawn on whether he took Viagra as research for his role. It was difficult enough infiltrating the cut-throat world of pharmaceutical sales – a closed shop to outsiders.

‘I met with doctors and pharmaceutical reps because I was interested in how Viagra is sold, but it was really difficult,’ he says. ‘No one wanted to say too much, particularly as jobs are so scarce nowadays. It was much more secretive than I thought it was going to be.’

Gyllenhaal says he finds it difficult to get excited about movies that don’t stir things up.

At 29 he’s best known for intense and innovative films – ones that have also been commercial successes, raking in a total of £1 billion.

There was his breakthrough cult hit, the surrealistic Donnie Darko. Rendition, with Reese Witherspoon, was about the controversial practice of interrogating terrorist suspects abroad. And the Oscar-winning Brokeback Mountain focused on the doomed love affair between two cowboys, Gyllenhaal and the late Heath Ledger (with Anne Hathaway playing the former’s wife).

When he does turn his hand to the odd action blockbuster, such as this summer’s videogame adaptation Prince Of Persia, he goes all out, transforming himself into a rippling slice of beefcake quite at odds with the shuffling nerd of Donnie Darko.

In real life, Gyllenhaal could still pass for an athlete. He’s 6ft tall and looks fit in faded jeans, brown boots and a black T-shirt stretched over a muscular chest and broad shoulders.

‘I used to go to the gym. I don’t any more. I just like being active, breaking a sweat. If someone says, “Do you want to go and hike this mountain?”, I’ll just go and do it.

‘I don’t have children, so I can still be selfish and say, “I’m going to go and be on my bike for three hours.” I ride mostly Trek bicycles – a Madone. I ride in Manhattan. You usually get where you’re going faster, but I’ve gone to many a big meeting sweating, with people thinking I’m a creepy person.’

‘Lance Armstrong saw me talking about bikes in a magazine, called me up and said, “You want to go for a ride?” And that was that – we rode. I took lots of caffeine and those energy packets and just went for it. I think I may have been sick the first ride I went on with him. I kept up with him, but I don’t think Lance was pushing very hard.

‘I rode L’Alpe d’Huez, which is one of the stages on the Tour de France, and that was a pretty intense climb, one of the best rides I’ve had in my life. And there’s nothing more beautiful than riding through redwoods in Northern California, cutting in and out of thousand-year-old trees.

‘I definitely have an alpha-male aspect to me, because if I see someone up a long way ahead I always try to catch him; but mostly I just like the solitude. It’s something my father instilled in me when I was a young boy. In the morning before school he would wake me up and we’d go on runs together. I still run – almost barefoot at the moment – just enjoying being physical, being active and being outside.’

It’s evident the self-contained actor is more content on the trail than schmoozing at celebrity parties. Gyllenhaal is actually quiet and introspective by nature.

Late last year he broke up with his long-term partner Reese Witherspoon, but Gyllenhaal won’t discuss the split, or the fact that he’s now reportedly dating pop star Taylor Swift, though he does tell me, ‘Family is ultimately what I want. That’s all that matters to me in the end.’

He also says that friendship is important. The untimely death of his Brokeback Mountain co-star Heath Ledger in 2008 affected him deeply.

‘I do miss him…’ Gyllenhaal stops mid-flow, looking emotional.

‘There was an intimacy that we all shared on that film – I’d never experienced anything like it. It was just us by this river in Calgary for a month, in the middle of nowhere. There were no hotels to stay in, so we lived in trailers out there in the wilds. We would make coffee together in the morning, eat together and sometimes we’d ride horses to work. We were out there alone and all we had was each other.’

Gyllenhaal is godfather to Matilda, Ledger’s daughter with their Brokeback Mountain co-star Michelle Williams. He, too, is from an all-Hollywood background – together with his older sister Maggie (Oscar-nominated for her role in Crazy Heart).

Their father Stephen Gyllenhaal is a director, and mother Naomi Foner is an Oscar-nominated screenwriter. Both fostered their children’s dramatic aspirations.

‘We lived on the east side of LA between Koreatown (a poor ethnic neighbourhood) and Hancock
Park (a fashionable area). Both my parents were very socially conscious. Growing up, my mother was always involved in charity work and she would get us involved in places like the homeless shelter.

‘They were black sheep in their families. My mother’s parents were doctors. My father came from a very small town in Pennsylvania. All either of them wanted to do was to write and make movies. Our home was like a circus with various people coming in and out. (Oscar-winning director) Steven Soderbergh lived in the room above our garage.’

Gyllenhaal has particularly fond memories of Paul Newman.

‘I was 14 when my mother got to know him as she was writing a screenplay for him. I knew him better from salad dressing, which is the great irony of different generations! My mum had a meeting with him and he said, “Come and meet me on the racetrack, and bring your son.” We went and he put me in a race car that was all souped up, drove me onto the skid pan, spun the car around and taught me what not to do. It was extraordinary.

‘I am a little bit Buddhist – in that I believe, in some way, that you choose your family. Convention was never something my family were interested in.’

Few people will be in any doubt of that, when Gyllenhaal – all of Gyllenhaal – appears in Love & Other Drugs on December 29.

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