Jared Leto: 'I road-tested my character to get a little judgment, some meanness, a little condemnation'

Jared Leto, this year's odds-on favourite for the best supporting actor Oscar for his role as the drastically emaciated transgender Aids activist Rayon in Dallas Buyers Club is recalling the first time he "road-tested" Rayon in public. "You kind of have to register reactions and things, so inevitably the day comes when you take her out for a walk: shaved, waxed, wigged-up, the whole bit. To get a little judgment, some meanness, a little condemnation was a useful thing for the part. I went to Whole Foods Market to, I guess, just stare at the food, because I certainly wasn't eating a whole lot of it. And I got three distinct looks from people. One was, 'What is that?' The second one was, 'Who is that?' and the other was, 'I dunno what the fuck that is but I don't like it.'"

This is not the reaction that the actual, for-real Jared Leto generally provokes. To play Rayon, he shed some 40 pounds of body weight, not for the first time, but today it's all back on and as usual he looks better than anyone for blocks around, as young as he did two decades ago as Jordan Catalano in his breakout TV show opposite Claire Danes, My So Called Life. Just arrived in town from touring with his successful rock band 30 Seconds To Mars, he's under the weather from a head cold exacerbated by jet lag, but those blue eyes still burn brightly.

Dallas Buyers Club has been building steady buzz since its premiere at the Toronto film festival last year, and the best supporting actor talk has only grown since then. The Screen Actors' Guild and Golden Globes wins are already his. The film tells the story of Ron Woodruff (played by Matthew McConaughey, who lost even more weight than Leto and looks almost agonisingly skeletal), a blue-collar rodeo gambler and ne'er-do-well who finds himself diagnosed with Aids in 1984 and is given 30 days to live by his doctors. As a conventional 1980s Texas homophobe, he's disgusted to be associated with gays, but this is serious stuff, and he finds the government and the medical establishment are glacially slow in addressing the epidemic. Finally, frustrated with the dangers of early AZT trials, he starts importing drugs not yet FDA (Food and Drug Administration) -approved, bones up on the latest compounds and cocktails of treatments for HIV/Aids, then establishes the titular Dallas Buyers Club to make them available to his fellow sufferers for a $400 annual membership fee. Woodruff's story is one of many valiant tales of the crisis years of the Aids epidemic featuring hitherto medically ignorant, deeply impatient patients taking matters into their own hands and sometimes making startling breakthroughs.

Born in 1971, Leto was just 10 when Aids made the covers of Time and Newsweek, and a teenager when the epidemic reached its gruesome zenith a few years later. "It was a very scary thing, especially for someone my age, just coming to sexual adulthood when it was really raging, it was frightening. I was living in all different cities on the East Coast at that time, and there was a panic. It was a very real thing back then, the idea that suddenly you could die horribly from the most beautiful thing in the world, having sex with another person. You wait your entire life to get going on sex and then suddenly it's a nightmare. It was like, but I just got here! It's good to revisit that time, it's really necessary, because our memories are so short."

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Leto's Rayon is a transgender woman – I tell Leto she reminds me of Vivien Leigh – whom Woodruff first meets in hospital when a circulation blockage in his leg causes him agony. Rayon, nothing but a weirdo and a pervert to the as yet unenlightened Woodruff, is in the next bed and offers a leg-massage. It's a wonderful scene – Woodruff goes from, "Get your faggot hands offa me" to, "Oh, that feels great..." and back again – and all the more affecting, perhaps, because the two actors met for the first time when director Jean-Marc Vallée called action on this moment.

"We didn't interact at all before that scene," says Leto. "I like those kind of games for the characters. Plus, how is it gonna help Matthew to know more about my character before he even meets me?"

Leto is famously an adherent of the total-immersion approach to his roles, which has occasionally led to problems on set. He has apologised publicly to Jennifer Connolly for his behaviour while making Darren Aronofsky's junkie nightmare Requiem For A Dream, but neither has ever specified exactly what that behaviour entailed. Often scorned, the immersion process is nonetheless a serious approach to a role, unless, as he says, "you come to this approach to acting from a position of conceit, or of narcissism, or with the wrong intention or a misconception, then it's a mess". Leto, a marvellous actor, has the awards to prove he's serious, of course.

"It's easy to scoff," he adds. "It sounds like magic or something corny but really the determinant, instead of talking about 'staying in character' should be focus, determination and commitment; staying in a place that's close, concentrating on things that need to be attended to. How many minutes of a given day on a movie set actually involve acting? Not many. So if you stay in that position, close to the part, it's like you get all this batting practice, this extra time; stuff like that is crucial. For a lot of people on the set, it ended up helping them as well. Once they were used to seeing me like that, after the first day really, it felt like I was Rayon to them, and that helped me persuade myself."

He also shaved every hair from his body. "The armpits were the worst," he remembers, "but when that first eyebrow came off, whoosh, in one stroke, that's something you really notice, and there's all those urban legends about how eyebrows never grow back! But they do grow back, it turns out."

It's five years since Leto's previous movie, Chapter 27, a film that didn't quite earn or properly showcase his amazing performance. He played Mark Chapman, the murderer of John Lennon, for which he took the opposite approach, gaining some 67 pounds, something he says he would never do again or advise anyone else to try. He sure didn't look like Vivien Leigh that time. "Really, it's a stupid thing to do. I got gout, and my cholesterol went up so fast in such a short time that my doctors wanted to put me on Lipitor, which is for much, much older people. Again, though, a fascinating journey. I had some reservations because I love everything John Lennon ever did. But in the end I wanted to explore a specimen embodying the total failure of humanity, just walk down this brutal path."

'For a lot of people now, it's not as big a deal to be working in a number of fields, and ultimately, results do speak for themselves'

And what happened to all that extra poundage? "I lost a lot of it by going on tour; not straight away, but I was still kinda waddling around onstage for the first few dates, and eventually it came back off, thank God."

That's what Leto's been doing in his long hiatus from moviemaking. 30 Seconds To Mars, which he founded with his brother Shannon in 1998, have been making records and a lot of money all over the world for the last five years. But the music business is even more of a jungle than the movie business, as he learned to his cost. His label EMI, under the terms of a slavish years-long contract, sued the band for $30m.

"They own you forever and that's why we fought them. We sold millions of albums and found out that they'd been lying to us about money. Once we sold three-and-a-half million albums and they didn't pay us a penny, not a nickel. And on top of that they claimed that we were 30 million in debt and they sued us for it! Should have been the other way around, really. It took us years to fight our way out of it." They filmed the whole ordeal, lawyers and all, and released it under the title Artifact (directed by Leto under the decidedly non-glamourous nom de plume Bartholomew Cubbins), a documentary which may depress a few aspirant musicians enough to hang up their Fenders and re-enrol in Bartenders' Academy.

The movie star versus rock star dynamic enrages some people, does it not? They'll let you be one or the other, but never both. "I think that is ebbing away nowadays," says Leto. "For a lot of people now, it's not as big a deal to be working in a number of fields, and ultimately, results do speak for themselves – if you're doing good work – so I think recently that perception has corrected itself. But it is just another of those weird, subtle pre-conditioned notions, prejudices or jealousies, even. It makes you stronger.

"And anyway, come on," he adds with a big lascivious smile, several months before the unhinged love-fest for Rayon begins in earnest. "How many people in the world do you need to love you?"

The Dallas Buyers Club is out on Friday