The splendidly named 33-year-old actor Benedict Cumberbatch looks absolutely dreadful. He is known to have "interesting" features – sloe-eyed and snub-nosed, with a sort of startled-meerkat-meets-a-Magimix look about him – but today he is also pasty-faced, with bottle auburn hair and a nasty, shiny cream shirt. His suit looks one size too small for him. It's a wet February morning on the Cardiff set of Sherlock, the BBC's modern adaptation of Arthur Conan Doyle's classic. Cumberbatch, who plays Holmes, twists and shifts about in his chair. He sits on his hands. He does impromptu imitations and funny voices. His eyes jag from side to side. Mark Gatiss, who co-wrote the screenplay, says, "Oh, he's always like that. You should hear him in the car on the way back to the hotel in the evening. Does impressions of everyone. Hilarious."
Months later, Cumberbatch is sitting on a balcony in London being grilled by a hot midday sun and three journalists. From behind a glass partition, I wait my turn and watch. He's a smoker. No wriggling now. He's talking fluently, gesturing, smiling, elaborating. It's soon clear that I got him wrong last time. He looked so odd because he was in costume. His clothes fit him now. And so, rather strangely, do his features. The face is interesting, and appealing, with changeable slants and angles. It is also, thanks to the sun, rather pink.
Most humans might want to leave this scene to take a rest in a cool, darkened room. But not Cumberbatch. He goes clomping off to the bathroom in an enormous pair of black leather biking boots. And then, a minute later, he's back. He is not tired. And, he says, he's not hyper. He tells me he has had two coffees, five cigarettes, quite a bit of water, some juice. He's fine.
Cumberbatch has a reputation for playing odd, brilliant men very well, and his Holmes is cold, techie, slightly Aspergerish. He talks 19 to the dozen and is forever coming up with lines such as, "From the tiny scratches on your mobile, I deduce that your brother is an alcoholic and that you don't get on with him."
Sherlock was, he says, a very enjoyable part, even if he did end up with pneumonia from the strain. "There's a great charge you get from playing him, because of the volume of words in your head and the speed of thought – you really have to make your connections incredibly fast. He is one step ahead of the audience, and of anyone around him with normal intellect. They can't quite fathom where his leaps are taking him. Zip zip zip. But you catch me in the car on the way home after and I am, 'Whoahhh!'" He does a theatrical wilt in his chair. I repeat what Gatiss said about him. "I do what? God! How exhausting. How fucking awful. I don't like the sound of myself at all. Well, mimicry is what I do, and a big part of what Holmes does, too. I love doing impersonations of people."
Did he learn any new skills for the part? "A bit of violin work – how to hold the bow, do fret work. And what he does. You can't help but cast an eye round you and think about people and the explanation that might lie behind the exterior show..."
Surely he can't believe that tosh? He shrugs as if admitting a vice. "A couple of times on trains... you can't help it. That indentation where a wedding ring should be, the dynamics of families. People in a moment of isolation, certain things do stick out. It's an achievable superpower."
Sherlock Holmes, Cumberbatch says, "makes you look at the world in the way you do anyway as an actor – as a rich canvas for observation. One of the fears of having too much work is not having time to observe. And once you get recognised, there is nowhere for you to look any more. You can't sit on a night bus and watch it all happen."
Until now, Cumberbatch has never been a household face. He has had small parts in big films – he played Paul Marshall (confectionery magnate and rapist) in Atonement – and big parts in small films, notably as the young Stephen Hawking in a BBC2 drama about the physicist, and in 2008 starring as a troubled mathematician sucked into a global conspiracy in the mini-series The Last Enemy. But Sherlock, created by two Doctor Who writers and probably destined for a larger primetime audience, could propel him into street recognisability.
So, too, could his next film. Cumberbatch is to play Major Stewart in Steven Spielberg's War Horse, based on the children's novel by Michael Morpurgo. After this interview, he is off for a riding lesson. He says, with great enthusiasm, "I charge in, Charge Of The Light Brigade-style, on this massive Spanish stallion with a sword out. I have a gladiator moment." He holds his arm out in front of him and does a jerky little swivel. "There! Horrible! That's how to disengage with what you have on the end of your kebab skewer."
Cumberbatch is one of the few British actors you might reasonably expect already to have a passing acquaintance with horses and things military. Like the Light Brigade's hapless Earl of Cardigan, he went to Harrow public school: his parents, the actors Wanda Ventham and Timothy Carlton (a stage name; he's a Cumberbatch, too), were very keen to give him the sort of high-octane education that might encourage him to consider not going on the stage.
''Having your adolescence at an all-male boarding school is just crap," Cumberbatch says, though he also seems to have been a model pupil and threw himself into extramural activities. He went paragliding and abseiling. He had an arts scholarship and painted huge oil canvases in a disused squash court. And, of course, he acted. His school drama teacher, Martin Tyrell, calls him "the best schoolboy actor I've ever worked with".
Cumberbatch went on to study drama at Manchester University. "I needed to be out of the danger of tying a cashmere jumper round my neck," he explains. "I wanted something a bit more racy, a bit more different, a bit more egalitarian. I had a thoroughly healthy – and unhealthy – mix of friends." He overdid things – he often does – and contracted glandular fever. But he had a ball.
After that, he did a one-year course at the London Academy of Music and Dramatic Art, and by the time he finished there he already had an agent. He has worked steadily ever since.
Cumberbatch is widely recognised as a stage actor, covering a repertoire from Ibsen and Shakespeare to the theatre of the absurd. But his big-screen breakthrough came in 2004 when he starred in Hawking and was nominated for a Bafta. It was such a startling performance that other odd geniuses, such as Van Gogh and Holmes, have since come his way. He also does a pretty good line in costume toffs – he played Scarlett Johansson's pompous husband in The Other Boleyn Girl and the vain young Edmund Talbot in a BBC adaptation of William Golding's seafaring epic To The Ends Of The Earth.
He says the impression people have of him is that " I just play neurotic, fey people who would have died with a cold compress to their head. But I do work on the variety. I do try. At the moment, I am playing a 38-year-old man whose chest is fucking huge," he says of his role as David Scott-Fowler in Terence Rattigan's After The Dance at the National Theatre. "He thinks in very predatory sexual terms. He is a child, like a lot of alpha men, but there is nothing of Van Gogh or Hawking about him."
Cumberbatch has multiple agendas. There is so much he wants to do. He is desperate, he says, to paint again. He and his girlfriend, the actor Olivia Poulet, would like to have children. He wants to write. He really is the ultimate thirsty young man.
Even negative experiences are grist to his mill. Asked about the time he was carjacked in South Africa with co-stars Denise Black and Theo Landey while filming To The Ends Of The Earth in 2004, he lights up a cigarette, fixes his eye on a point on the table and he is away. He sets the scene – he's been on a shoot and it's night-time on a highway near the Mozambique border. A tyre blows and they have to stop. The juggernauts whizz by in the freezing African night. The grassland stretches out seemingly for ever. Then six men appear. Cumberbatch and his two colleagues are held up against the car. The men utter terrible threats and tie the trio's hands behind their backs with their own shoelaces. There follows a very intense sense of death being close by. He describes the running commentaries in his head and the strange moments of calm. He remembers the Radiohead song playing on the car sound system and recites the lines.
Then things get worse. He is thrown into the boot of the car, and later on to the ground. He can feel a trickle of blood from the side of his head and insects scrabbling away. Even though they are bound up, he can squeeze Denise's hand. And he keeps thinking, "I wish I knew morse [code]" then he thinks, "Hmmm, that would be fucking irritating and useless if she didn't know it, too."
And so on and so on and so on. He isn't just telling, he is reliving the experience. The car-jacking lasted two and a half hours; this is a mere 26 riveting minutes. Finally, we reach the aftermath, when Cumberbatch went on an "adrenaline junkie drive", skydiving, hot-air ballooning and generally "looking over the precipice".
Now things are calmer. "I ride a bike, but I am not seeking man thrills on that level, no. There is a sense of impatience and a yearning for a life less ordinary, which is destructive, as it leads you away from harnessing the true value of things. But it also gives you fantastic knowledge. I know I am going to die on my own, which is something you don't realise until you are faced with that. You leave this world as you come into it, on your own. A sobering but profound thought to realise early in life."
His mobile phone rings. It's his driver telling him he's waiting down below. Must go. He shakes hands. He is off. Zip zip zip.
• Sherlock starts on BBC1 on 25 July.
