Benedict Cumberbatch is sipping what looks suspiciously like green tea, the drink of choice for Hollywood A-listers who take their health – not to mention themselves – terribly seriously. For a worrying moment, I wonder if the star of Sherlock has gone all LA on us.

“No!” he laughs. “No, I’m just being good today. And actually, you know,” he adds happily, “LA is not all that abstemious. There’s much more drinking and partying going on than I thought there would be. It’s all about extremes with that place, so you get the worst excesses of everything – the health-kick thing, over-indulgence, recreational drug use, everything. It’s paradise!”

Since the hit BBC drama Sherlock transformed the actor into a superstar two years ago, he has appeared in Oscar-nominated movies War Horse and Tinker Tailor Soldier Spy, spent the first half of this year in Los Angeles filming the blockbuster sequel to Star Trek, is about to head off to New Zealand to make The Hobbit, and has inspired a global following of adoring fans who style themselves on Twitter as the “Cumberbitches”.

But the classically trained actor also delivered an Olivier award-winning performance in Frankenstein on stage at London’s National Theatre, and tonight appears in a major BBC period drama series, Parade’s End, an adaptation of Ford Madox Ford’s quartet of novels published in the 1920s.

Cumberbatch may have soared into the celebrity stratosphere, but his feet are still firmly on the ground. Earlier in the summer, he passed up the chance to see Andy Murray in the Wimbledon semi-final – “the A-list thing to do” – in order to read the lead in John Osborne’s watershed 1956 play Look Back in Anger in a London theatre. “So I’ve not completely sold out,” he grins.

Nevertheless, at 36 he has become what his industry likes to call "hot", so I wonder if he feels his stardom was a long time coming or does he think, “How on earth did this happen to me?”

“Neither really. I had my opportunities to do commercial stuff before Sherlock. I’ve just chosen, very specifically, to do what I do.”

Before Sherlock, Cumberbatch had been enjoying a critically acclaimed, if not wildly glamorous career, starring in TV dramas - Tipping the Velvet, Hawking, Small Island - appearing on stage at the Royal Court and the National, and playing supporting roles in mainly British films – Atonement, Starter for 10, Four Lions.

“To be honest, I have no regrets, because I’ve loved the journey I’ve gone on. I was doing exactly what I wanted to do, landing lovely roles and getting the respect of my peers. A few people would go, ‘Oh, I loved you at the Royal Court,’ but I was just quietly getting on with it.” Then Sherlock happened and he couldn’t walk out of his front door without being mobbed.

“Overnight – I mean, it was just extraordinary. I remember being really quite scared at the Twitter thing. I literally did expect [someone in] a helicopter with a Swat light to abseil down over my house. It was just, like, ‘Whoa, the whole world is suddenly interested, and a bit obsessed.’ Just because I’m in my 30s, it doesn’t make the weirdness of no longer being private any less. I don’t think it matters whether it happens when you’re 25 or 55, it’s just very odd. Something is suddenly taken away, and it’s weird.”

Cumberbatch is so engagingly unguarded, and cheerfully forthcoming, it would be easy to forget just how big a star he is, for there is none of the bland wariness of a Hollywood commodity in his manner. He finds himself at a particular moment in his career where he’s successful enough to know what proper traffic-stopping, mayhem-inducing fame is really like (“Walking through central London on a Friday night is not top of my list to do any more, I have to say”), but not yet jaded enough to stop marvelling at it.

“It’s just really bizarre. [My friend] James McAvoy was walking through Leicester Square and this big guy just picked him up and licked his face. Can you imagine? And that’s not,” he adds, starting to laugh, “because he’s short and lickable – he’s not got a lollypop head. It’s only because he’s on the telly.”

When Cumberbatch performed in Frankenstein on stage, he was unnerved to see the same faces in the front row, night after night. “I was like, this is weird. And I told them that, and they were mortified. I said, ‘But come on, look, just hear me as one of you. Because I used to be in the audience, I used to obsess about things. But I don’t understand this.’ Two young women from China saw literally every performance. And I asked them, ‘How do you afford the time and the money?’ And they just said, ‘Oh it doesn’t matter. We love you’.”

Cumberbatch hadn’t been prepared for his surprise promotion to the status of Hollywood heart-throb. On screen he has the sort of face that critics tend to describe as interesting, rather than beautiful – though in person he’s far better looking than I’d expected, reminiscent of a young Hugh Grant. I‘d wondered if he might be touchy about discussing the improbability of his becoming a pin-up, but instead he bursts out laughing.

“I find the whole thing hysterical. I see the same problems I’ve always seen when I look in the mirror. I don’t go, ‘Hey, Brad, looking good today’.” He mockingly kisses his imaginary reflection, then shakes his head. “No. But people are suddenly mad about it. But I think, seriously, it comes a lot with the work, because still, if you put me in line with Brad and George and all of them, you do kind of go,” and he mimes pointing along the line, “Lovely, yes. Handsome, yes. Oh, strange-looking guy. Next, lovely, yes. Handsome, yes...” He breaks off, laughing again. “And why shouldn’t you? But I’m quite happy with that, because hopefully it gives me more of a shelf life. And it means I can still play character roles without people going, ‘Oh, he’s doing a Charlize Theron – he’s doing beautiful playing ugly’.”

In his personal life, it must be quite strange at 36 to find his sexual currency inflated so dramatically. “Well,” he agrees, “it puts a bit of a spring in your step. It’s nice, you swagger a little bit, it’s enjoyable. But think about it – the actual reality of, ‘Will you pull every beautiful woman in the room every night?’ Nah. They come up to you and go, ‘Oh. Umm, maybe.’ The point is, I can’t take it seriously, because it’s all through a filter of them knowing much more about me than I know about them. So yeah, it’s kind of weird. But it’s to be enjoyed. I’ve punched well above my weight this year. And that,” he adds coyly, “remains very much a secret.” If only his 15-year-old self could have known what was coming, he reminisces wistfully. “If I only knew.”

Some men who become famous resent the fact that the world suddenly treats them like an Adonis when it never used to. “Yeah, that’s me,” he jokes, with heavy irony. “I hate the fact that I’m a sex symbol. God damn my beauty and success. No, I find it hysterically funny, I really do. It’s a giggle. I wield it with a massive smile.”

The only thing he worries about is finding his private life splashed all over the tabloids. “If I have a lot of fun with it, then there’s far more chance of me being exposed having a lot of fun with it, which makes me look like a w****r. That is a little bit of a worry for me. It inhibits me from going, ‘Dah-dah-dah, let’s party!’ because you just know your behaviour is going to get on some blog or tweet or gossip column. Before, with anonymity, I didn’t have to worry about that problem. But then,” he adds with a wry chuckle, “I didn’t have that much to wield with a smile.”

Cumberbatch broke up with his partner, the actress Olivia Poulet, early last year. The couple had met at university and been together for 12 years, so assumptions were made that the relationship hadn’t been able to survive his new fame. In fact, he says, it had run its course before Sherlock came out, and the break-up had nothing to do with work. “As much as people have conjectured, ‘Oh, it’s typical, it’s the first love being done over,’ well no, of course it’s f***ing not.” For a rare, fleeting moment he looks annoyed. “But you know, everyone gets hold of scant facts, and because they’re obsessed with me, they weave a narrative.”

Nevertheless, his life does feel a bit like a two-part drama – Before Sherlock, and After Sherlock. “So I know there’s a Sherlock audience that will come to Parade’s End and scratch their bonces quite early on, going, ‘Is this really what Sherlock did next?’ Because it’s an utterly different character in an utterly different world.”

Adapted by Tom Stoppard from the four novels of Ford Madox Ford’s Parade’s End, Cumberbatch plays an anguished Edwardian aristocrat. Taciturn and tortured, he certainly doesn’t – unlike his Sherlock – talk at a million miles an hour. “There’s none of that hyper-articulate mental vomiting. I think people may think, ‘Well why would he want to do that, because it’s nothing like Sherlock’. But that’s exactly why.”

And his role in Parade’s End may not help Cumberbatch with the only other matter that seems to bother him, namely “All the posh-bashing that goes on.” Both his parents are actors – “I wasn’t born into land or titles, or new money, or an oil rig” –  but they sent him to Harrow, and as a consequence he says he’s been “castigated as a moaning, rich, public-school b*****d, complaining about only getting posh roles.” As he’s one of the most gifted, intelligent and likeable actors this country has produced, I’m not surprised he can’t even be bothered to engage the attack.

“It’s just so predictable,” he sighs wearily. “So domestic, and so dumb.” I just hope he’s not serious when he adds, “It makes me think I want to go to America.”

Benedict Cumberbatch stars in Parade's End - tonight at 9:00pm on BBC2

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