Pin-up who is hard to pin down

Andrew Scott's career has been as unpredictable as his portrayal of Moriarty in Sherlock Holmes. It's good to keep reinventing yourself, he tells Alice Jones

An arch-villain who blows raspberries, has "Stayin' Alive" as his ringtone and introduces himself to his enemy with the words: "Is that a British Army Browning L9A1 in your pocket – or are you just pleased to see me?" It's not quite what you'd expect from Moriarty, the man that Sir Arthur Conan Doyle described as "the Napoleon of Crime, the greatest schemer of all time, the organiser of every devilry, the controlling brain of the underworld, a brain which might have made or marred the destiny of nations". But then little about the BBC's zesty reimagining of Sherlock Holmes for the 21st century is predictable. And Jim Moriarty, as played by Andrew Scott, is one of the least predictable and most thrilling elements of all.

His "consulting criminal" is the definition of mercurial. Slippery and highly toxic, one minute he's a slick gangster in a Westwood suit and aviators, the next, a fidgety bag of neuroses, the next, a bellowing fiend. That's what makes him so terrifying, says Scott, in real life an affable 35-year-old from Dublin. "People that you're really frightened of, it's usually because you don't know anything about them. You think, 'God, I've no idea what he may do'." Slight, with dark hair and puppyish eyes, he doesn't look like an arch-villain. "You don't need to have humps and fake noses and wigs and funny teeth," says Scott. "What is really scary about Moriarty is that he can be right in front of you, hiding in plain sight. There's an audacity about that."

The actor didn't spend much time reading the original stories or watching previous interpretations. "I'm not a big guy for research. You've got to take a risk. With Moriarty, there was pressure because he's such a famous character but there's no point in me trying to copy somebody else."

Indeed. Scott's menacing performance has already won him a legion of fans. In the summer, when he was appearing in the Ibsen epic Emperor and Galilean at the National Theatre, they would mob him at the stage door. At a recent preview screening of Sherlock, the atmosphere was "nigh-on hysterical", he says. As Sherlock, Benedict Cumberbatch has his band of female followers – the Cumberbitches. Is there a Moriarty mob? "Yes. Mainly girls, aged between 17 and 25, with a bit of dark edge." He giggles, nervously.

It's remarkable when you consider that Moriarty has only appeared on screen for a total 15 minutes so far. This Sunday, after a hefty build-up including one series cliffhanger, an extended swimming-pool stand-off, one hound-related hallucination and quite a lot of menacing text messages, he will have his showdown. The last episode, "The Reichenbach Fall", will see him attempt the heist of the millennium, and perhaps destroy Holmes once and for all. The 1893 story on which it is based, The Final Problem, ends with both falling to their deaths in the Alps – or perhaps not. Will there be a third series? "I'm not sure yet. I'm sure they're talking about that at the moment."

Whether Moriarty lives to plot another day, Scott is keen not to become typecast. He's currently filming The Fuse, a gritty BBC four-parter, playing a melancholic detective opposite Christopher Eccleston's corrupt councillor, and an adaptation of Daphne du Maurier's The Scapegoat with Eileen Atkins in which he plays a "terribly, terribly posh person with a side parting".

On TV, he's recently popped up playing Paul McCartney in Lennon Naked, and a mannered 1950s actor in The Hour as well as in American blockbusters like John Adams and Band of Brothers. On stage, he is most proud of the work he's done with new writing – including Simon Stephens' Sea Wall, Mike Bartlett's Cock and David Hare's The Vertical Hour on Broadway.

If you wanted to find a common thread, you could say that Moriarty is the latest in a long line of hard-to-pin-down characters. This playful fluidity saw him star in two theatrical ménage-à-trois in a row recently – Cock and Design for Living. A film of Chekhov's The Duel, produced by ex-Merchant Ivory president, Donald Rosenfeld, out later this year, makes it a hat-trick of love triangles. "I do sometimes play characters that are a bit ambiguous. You've got to be brave about that sort of stuff. I like the sense of people not feeling too secure, not immediately knowing what they have in front of them."

Intense and highly strung on stage, off it he's witty and laid-back. "I don't think I'm intense in life. This must be where it comes out. I don't really like anything to be too serious. Then you lose the humanity of it."

He was born in Dublin, where his family still live. His older sister is a sports coach and his younger sister is just starting out as an actress. When he was growing up, his father worked for an employment agency and his mother was an art teacher. Scottwas about to go to art college when he won his first role in the Irish film Korea, aged 17. He still paints – "figurative stuff, people mainly. I was good" – and would like to train. He has lived in London for the past decade with his partner, who is "sort of" in the business. He clams up. "It sounds maybe a little old fashioned, but the parts I want to play and I do play, you don't want to inject too much of your own personality. What you sacrifice then is a slight mystery."

Red carpets, interviews and social networking are all alien to him. When a friend told him that he was trending on Twitter following his debut as Moriarty, he made a rare foray online. "So terrifying," he says, burying his head in his hands. "I would never go on it again. Oh God, it just made me want to go to sleep for three weeks. People said lovely things but then people also said the most vicious, horrible things."

When "The Reichenbach Fall" airs, there is likely to be a flurry of new tweets, but this time he is ready for the attention. "There's no doubt that it has increased, but you can be as recognisable as you like." Like Moriarty? "Exactly! Just like Moriarty. You can make yourself invisible if you really want to."

Sherlock: The Reichenbach Fall is on Sunday at 9pm on BBC1