Hoult… who goes there?

"Oh no. Interview, uh oh. I always panic about these things." Nicholas Hoult does look genuinely apprehensive, which is annoying because he's been perfectly at ease until now. He arrived promptly at the west London studio where he's to be photographed at 9am on this December Monday morning, introducing himself very politely to everyone. He swapped jeans and hooded top for a dapper suit by Tom Ford, the fashion designer whose directorial debut, A Single Man, stars Hoult alongside Colin Firth and Julianne Moore, then chatted away with the make-up artist about his newly shorn head, a crop to cut out hair extensions for his role in upcoming CGI blockbuster Clash of the Titans. But now he's staring at me like a rabbit in the headlights, albeit one that's 6ft 3in and wearing an expensive brocade jacket.

I say reassuring things, but he looks unconvinced. Why do interviews make him panic? He frowns and seems to think hard. "Dunno," he finally offers, unhelpfully, but at least he sits down. Unfortunately we're on a sofa in the studio dressing room which is so tiny that we could, as Hoult says later, have spent our time giving each other a foot massage. He hunches, like he's waiting outside the headmaster's office.

Hoult's reaction is sensible really. No one wants to be crammed on a sofa and asked annoying questions about how he feels and what he thinks, but it's surprising that an actor this experienced hasn't found a way to deal with the experience by now. Though we meet the week before his 20th birthday, he's worked in film and TV for years. He was spotted in a theatre audience aged three, by a director who suggested to Hoult's mother that if he could concentrate on a play at that age then he could probably be in one. He's been acting ever since, becoming properly famous at 12 as geeky schoolboy Marcus opposite Hugh Grant in the 2002 adaptation of Nick Hornby's About a Boy. He's co-starred with Nicolas Cage, Michael Caine and Kenneth Branagh, and became a star in his own right at 17, thanks to Channel 4's drugs-and-sex-drenched teen drama Skins. His portrayal of heartless heartthrob Tony Stonem, full of callous swagger and vulnerability, was the show's standout performance. A billboard advert featuring Hoult's naked torso and his frequent sex scenes meant he also won many fans who weren't particularly interested in his acting ability. Last year, when he appeared in the West End in the play New Boy, adapted from William Sutcliffe's 1996 novel, the show sold out so fast that it was dubbed "Hamlet for the Skins generation", comparing Hoult's appeal to David Tennant's.

Chris Weitz, who co-directed About a Boy and produced A Single Man, thinks there are several reasons for Hoult's transformation from child actor to star: "Arbitrarily, he turned from the shrimp who came up to my armpit to the hunk who towers above me, and while his looks have matured, they've remained inherently interesting and unconventional. On his own, though, he's shown that the naturalness of his performance as a 12-year-old was not a fluke. He's willing to do a tremendous amount of work to get a performance right."

Hoult hated the attention Skins brought. Once, after his car was surrounded by people staring in, tapping on the windows, while he waited for his little sister outside her school, it even made him doubt he was cut out for acting. "I'd always had two lives before then – acting and the real world – and it all bled together. But it's interesting how quickly people forget. They look at me now, think they might know me, but then just walk on, which is brilliant."

This anonymity is unlikely to last: Tom Ford's A Single Man is already getting a lot of attention. Adapted from Christopher Isherwood's 1964 novel, it follows 24 hours in the life of British college professor George Falconer, played by Colin Firth. Falconer is lost in grief over the death of his partner, Jim, eight months before, and considering suicide. This day could be Falconer's last, which gives every mundane task and chance meeting vivid depth; or, as Falconer puts it, it's full of moments of clarity which allow him to feel rather than think. One of the people Falconer encounters is Kenny (played by Hoult), a student at the Californian university where the professor teaches. Kenny pursues Falconer relentlessly, offering him drugs, friendship and opinions. He's a sexy yet sexually ambivalent character whose motives are interestingly hard to read.

Both the film and Hoult's role in it could have easily misfired. Tom Ford is one of fashion's biggest names. As creative director of Gucci in the 1990s he turned the ailing label into one of the most successful of the decade; profits rolled in (the brand was worth $4.3bn by 1999) and Ford's ad campaigns – which included pubic hair shaved into the label's signature G and a naked, writhing Sophie Dahl – became emblematic of the times. But fashion is one thing, film is another, and the announcement that Ford wanted to direct was met with derision. He couldn't get studio backing and had to finance the film himself. For Hoult, this was his first grown-up role, where he plays neither son nor schoolboy. A lot rode on the film's reception, but ever since the 10-minute standing ovation at its Venice Film Festival premiere last September, there's been nothing but praise. Tom Ford, Colin Firth and Julianne Moore may be among the Oscar nominations next week, and Hoult's first adult role is a triumph. He must be very pleased?

"It was very good of Tom Ford to let me be in it," Hoult mumbles on our cramped sofa, looking as if I've spoon-fed him cod liver oil, "when he had all those great actors involved."

Luckily, Ford is more enthusiastic: "Nick performs in a way that seems effortless, subtle and honest. He has a maturity and depth that are remarkable." Ford particularly admired the way Hoult worked on a pivotal scene where Kenny takes Falconer on a midnight skinny dip, reviving the professor's sense of joy. Hoult had an eye injury caused by ash from the LA bush fires that raged during their shoot. "He insisted on working while he was in pain. He is a real professional. I can't say enough good things about him."

Hoult's first Hollywood film was the unappreciated The Weather Man with Nicolas Cage, which he shot at 15; to have made a second at such a young age is impressive. Would he be interested in doing more? Are there other directors he wants to work for?

"Um…" Hoult falters. "There are so many… Sorry, I've put all my energy into the first five minutes of this interview and now I'm crashing and burning. Can you write at the beginning that this was very early on a Monday morning?"

He does seem frazzled rather than purposefully difficult, and happily admits he finds it hard acclimatising to being a full-time actor. "I went through a mini-nervous breakdown when I left school [at 17]. It's a scary life and sometimes you think you'll never work again."

So what do you do when you're not working?

"Oh," he frowns and his impressive eyebrows plummet. "I potter around. You just have to go with it. My mum always says work goes in waves: you have a good spell and then it dips."

And his mum, Glenis, knows what she's talking about. Though she's a piano teacher and dad Roger is a retired airline pilot, all four of their children are actors. Hoult's older brother James lives in America and his sisters, Clarista and Rosie, have worked in TV and film. He even has pedigree: his great-aunt was Dame Anna Neagle, the biggest British female film and stage star of the 30s. Tom Ford thinks Hoult has been "beautifully raised"; he still lives at the parental home in Wokingham, Berkshire, because he's away so much for work that he likes to see everyone, friends and family, when he can. He stayed in London for the two-month run of New Boy, but used to worry that he'd be mugged walking home. "I'd try and think up stories that I could tell a mugger, like that I'd already been mugged or I had no money. You think, 'They've got to have a heart.' Maybe that's what I do when I've got spare time: make up scenarios where I sound cool."

Actually, being cool seems a low priority for a 19-year-old who could presumably go to any club opening or party he chose. He prefers the pub with friends from Wokingham – though he did apparently snog Pixie Geldof at her 18th birthday party and pulls a face when reminded of that. "It's awkward going to parties with people you don't know, especially when they think they know you. It was difficult after Skins because people thought I was my character, but as you've probably figured out by now I'm not very interesting… Oh this is all going horribly wrong."

It's not going horribly right. I wouldn't mind if he didn't seem so unhappy; it would be easier if he was rude – but he isn't. Well, not really. He enthuses about Clash of the Titans, in which he has a supporting role as soldier Eusebius, saying that everyone looked after him on set because he was the youngest. I ask if he ever gets tired of always being the youngest in a film cast. "Am I looking forward to being older? Not really. I'm not scared of growing up, but it just happens, doesn't it?"

Ford said he found Hoult "quiet at first, but once he's comfortable he's hysterically funny and quick witted." I'm obviously failing to make him comfortable so I promise I'm going to find some interesting questions, and he watches as I skim through my notes. "You've got piles of questions – they can't all be for me. Are these just generic ones you print out for everyone?"

They are all for him. Like most actors who've been working for years and have an ardent teenage following, there's heaps of information about him on the internet. But when he finds out that I've read that he used to have the McFly back catalogue on his iPod, that his favourite restaurant was once Nando's and that he formed a band with friends and covered Arctic Monkeys songs when he was younger, he accuses me of "haunting him with the past". But somehow the interview has reached such a level of awful that it's funny. When he's not holding his head in his hands, he's laughing as much as I am. In the end I ask him if he'd like to do more theatre, but the question seems so inane that I ask in a sing-song voice which makes him chortle. "You can't ask a question like that – what was that delivery?"

Well, I explain, it sounds like a competition winner's question so I asked it like a competition winner. "In that case, yes, I do want to do more theatre. It's fun, it's exciting and it's a real rush." His answer is a perfect performance of an insincere actor faking enthusiasm.

Doesn't it get dull saying the same words every night? "No, because you improvise – especially with Shakespeare. People really appreciate it at the end of Hamlet."

Maybe he would like to make some stuff up. He has in previous interviews – most memorably that in the second series of Skins the cast all had superpowers and that he was engaged to Kaya Scodelario, who played his little sister Effy on the show.

"I had to stop because everyone believed me. I started lying after I read this brilliant Michael Gambon quote. An interviewer asked him about his sexuality, about being straight. He said that he used to be gay, but gave it up because it made his eyes water. Saying things like that stops you going brain dead, but I only really do it if I'm asked a question I want to get round."

OK, do you have a girlfriend then? "No."

Is that a lie? "No." He looks tremendously serious and I'm reminded of what a good actor he is.

What do you want for your 20th birthday next week? "I'll be in New York – I never seem to be at home for my birthday any more – so I've told my mum not to get me anything."

Why not?

He stares at me with such a powerfully mournful expression that I flinch. "There's nothing that's going to bring me happiness."

Then he bursts out laughing. "That's perfect, you've got to use that." He's thoroughly entertained. So, of course, this is when the photographer asks for him, and he's off with a big smile on his face. I watch the shoot for a while, annoyed that Hoult is utterly relaxed for the camera, before I say goodbye. "You're going?" He looks put out. "Oh fine, knock off early. Though I was thinking, could you add in that I'm a bare-knuckle fighter?"

I next talk to Hoult six weeks later and A Single Man has been nominated for oodles of Golden Globes and Baftas, and Hoult's up for the Bafta Rising Star Award. The American premiere fell on his 20th birthday and he was given a cake at the afterparty, which was held at Vanity Fair editor Graydon Carter's New York restaurant, Monkey Bar, and attended by Madonna, Anna Wintour and Courtney Love. But when we speak on the phone and I ask about New York he just says it's nice to be home.

"I've been thinking about this interview a lot – I've had sleepless nights over it, actually. It was awful and I'm sorry. I've realised why I don't tell the truth in interviews. It's because they're printed months later, and you change so quickly – you have new thoughts, new everything – so people are reading an old version of you. As an actor, without sounding like a knob, everything's very self-obsessed. Everyone always talks to you about what you're doing and you're thinking about how to be someone else [a character], and you feel like you're in a bubble."

He says he does find doing press difficult and is a bit talked-out about A Single Man. "The questions I was asked most often were about Tom Ford's direction, playing a gay character and then there was the being-very-naked question [about his skinny-dipping scene with Firth]. It was just naked, gay and Tom, really."

He thinks anyone reading this will think he's sad and miserable, but I hope they don't, because you rarely meet someone who goes from being entertainingly awful to awfully entertaining, and struggles to answer questions months after they're asked. I check to see if he still wants me to say he's a bare-knuckle fighter.

"Oh yes – I've been training with my nan this morning," he laughs. "Actually, I've got tonsilitis and I've spent the last few days on the sofa eating ice cream with my nan. Today we're going to watch Driving Miss Daisy. Then we're going to re-enact it afterwards. We're already dressed up and… no, I'm not going to go there."

He's also been reading at lot. At the moment it's The Five People You Meet in Heaven and before that it was The Life of Pi, which he thought would make a wonderful film. "But I looked it up and Ang Lee's already on it. Damn him, he steals all my ideas."

He's very pleased with A Single Man's multiple award nominations. "A lot of films are judged by how much money they make but A Single Man isn't going to be one of them. It'll be judged by how it affects the people who go to see it and it's great to see it receiving attention, especially for the people who put all that hard work into it, like Colin and Tom."

And you, I add. "Oh, I didn't put that much hard work in; I did it in front of a green screen in my bedroom."

Director Chris Weitz said Hoult has "a well-developed sense of the absurd which keeps him from believing his hype", which perfectly describes the way he talks about himself. Hoult stars in the new ad campaign for Tom Ford sunglasses – a real coup in fashion – but he says he was hanging around in LA and thought he might as well do it. "It was just half a day with me and model Carolyn Murphy wearing sunglasses. Well, I wore sunglasses and Tom Ford clothes and she was pretty much naked. So it wasn't a bad way to make money."

He's so proud of his Bafta nomination that it's the only thing he doesn't make a joke about – not least because the Rising Star award was founded in 2006 to honour casting director Mary Selway, mother of Hoult's long-time agent Kate Buckley. "That makes it extra special for me and Kate. I was nominated for a couple of things for About a Boy, but I was younger then and quite nonchalant. Now I know you need to appreciate these things."

He has lots of work lined up: an indie film and a project that will keep him busy until the end of the year. He won't say what because he's superstitious and doesn't believe they'll happen until he's actually on set (though rumour has it he's been cast in Mad Max 4).

What else is he looking forward to? He thinks for a while before nominating the Miss World competition.

Really? I ask, what's your favourite round – swimwear or talent? "Talent, probably. Though wearing swimwear is a talent, too… oh I don't know. I'm looking forward to travelling, doing interesting stuff. Strengthening ties with people I already know and love. I'm serious – but it sounds so cheesy. I think these things, but I just can't say them out loud. But I mean it, I really do."★

A Single Man is released on 12 February

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