Andrew Scott: A pin-up who is hard to pin down

Scott's career has been as unpredictable as his portrayal of Moriarty in the BBC's new 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 wheedling 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 really 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 a bit of pressure because he's such a famous character but there's no point in me trying to copy somebody else. I hate that idea. Life's too short. Some people might hate it and I'm sure some people do. But you've got to put your signature on something otherwise what's the point?"

Indeed. Scott's menacing performance has already won him a legion of fans. In the summer, when he was appearing in the three-and-a-half hour Ibsen epic Emperor and Galilean at the National Theatre, they would mob him at the stage door. One teenage fan from China saw the play seven times. "That's a lot of hours," murmurs Scott. At a recent preview screening of the new 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 young girls, aged between 17 and 25, with a bit of dark edge." He giggles, a little nervously.

It's remarkable when you consider that so far, Moriarty has only appeared on screen for around 15 minutes in total. 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 finally have his big stand-off. The last episode of the series, The Reichenbach Fall, will see him attempt to pull off 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 apparently 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 new 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".

"I'm desperate to do something funny next," he says. "I want to keep everything balanced. That's why it's important not to have too much fanfare. If one story becomes too hot then you can't forget it. As an actor you want to remain fluid."

Fluid is a good word for Scott. He has worked steadily ever since he dropped out of his drama degree at Trinity College, Dublin to join the Abbey Theatre 15 years ago, but he's still, somehow, tricky to place. 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'll happily tackle Ibsen but is most proud of the work he's done with new writing – including in the premieres of 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." Upon leaving his Jesuit boys' school, he rejected his degree after six months for being over-serious – too many lectures about the semiotics of noise, and not enough theatre. "Everybody was writing down 'noise' and I was thinking, 'Is this costing a grand-a-half a term?'" When one of his essays came back with the sniffy observation, "Your idea of a paragraph is sadly deficient," he snapped. "I thought, 'Screw paragraphs! What's that got to do with anything?'"

Today, still, he has little time for thespian talk. He rarely reads his reviews, thinks the power of critics is "preposterous" and does not believe in working for the sake of art alone. "I don't like it when people ask actors to work for free – on the fringe – as if it's some kind of virtue. That annoys me – actors should be paid well." There is only one thing that makes him angry – noisy audiences. Someone brought a baby to one of his recent plays. "And of course, as is the wont of a baby, it cried all the way through. I thought it was incredibly rude. And rudeness to me is always linked to cruelty."

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. Scott had been 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 still like to train. He has lived in London for the past decade with his partner, who is "sort of" in the business. "And that's all you're getting." 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. It's an outlet for the angry. It's like going into a room and being punched, then kissed, then hugged, then kicked, then complimented and then slated."

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 it's weird, 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