Andrew Scott started acting young – and even more villainous than Moriarity. "I played Richard III in my mum's coat when I was 13. For the hump." Not that he has a taste for villains. "You really can't play evil. I see Moriarty as entertaining, funny, there’s a lot of comedy about him. Privately, I see things in the character that make him interesting: he’s many people at once. But pure evil?” He shakes his head. “I’ve been asked to play Satan, but I thought it would be a really boring part, no light and shade.”

There’s plenty of both in his witty, brilliantly malign turn as Sherlock Holmes’s foe: so it’s impossible for me not to ask whether Moriarty will be returning to our screens. “Hard to say anything...” he says, artfully diverting attention by taking a bite of his Caesar salad. “Let’s just say – um – that I’ve enjoyed everything they’ve given me to do.” I try again, rather hoping for that unnerving Moriarty glare, but he grins. However weirdly terrifying Scott was in his TV role, in conversation he is open and friendly, enthusing about work and laughing about the things he and I have in common, like a Catholic upbringing.

Scott, a Dubliner, was with the Jesuits at Gonzaga College in the 1990s. He acknowledges it’s been “a difficult time for the Irish Catholic Church, with the abuse scandals”. Did the revelations affect him? “They really did. You could sense it everywhere, a very troubling time, and hard for people like my parents. They had to question what they had  been told all their life.” On the other hand, the ritual of the Church has a real theatricality about it.

“Oh yes! Dramatic! And I was very much encouraged artistically: played rugby a bit but soon got into art and painting and acting – my mother was an art teacher. That aesthetic, artistic interest was very much supported by the Jesuits.”

It must have been less supported, though, to be gay in a religious school (he came out last year, without fuss). "Well, I didn't feel I was particularly different – everybody's sexuality is burgeoning at that age – and I wasn't bullied. But yes, I did find things I questioned, a definite unease. Disparaging things were said about homosexuality, absolutely. There was a biological, fear-based way of teaching sexuality. And...” he lightens with remembered glee, “sex education films! Earnest young actors always discussing sex and naturally deciding against it.” So, as a hungry young actor, he might have had a job doing those very films? “Indeed!”

As an openly gay actor, Scott has never felt typecast – unlike Rupert Everett, born two decades earlier, who’s been vocal about the professional handicap of coming out. In his new movie, Pride, which also stars Bill Nighy and Imelda Staunton, Scott plays Dominic West’s partner, a gay man shunned by his family and scared to return home. “Oh, I have no fear. The character in the film is not reflected in my own life at all.”

Andrew Scott Pride

He gives a touching performance in Matthew Warchus’s film (in cinemas today), which tells the true story of the gay community’s support for the striking Welsh miners in 1984. Scott admits that what he learnt by meeting with the real campaigners from that angry, bigoted time filled him with horror. “Oh, that wilful hatred gay people faced! Absolutely awful. But the film is about community, solidarity, the way that pain and suffering can make you reach out. When your burden becomes your gift and you learn empathy. Watching the film, I felt..." he pauses, looking for the right words, "satiated – and by something I didn't know I was hungry for."

It is a moment of such sincerity that we both pause. I felt something similar at the end of the film, but cynically thought the feel-good ending must be made up; surely the once-doubtful Welsh miners didn't really turn up with bands to lead the Gay Pride March? But they did. "It's a part of our history we should be proud of. We need to give up that 'them and us' idea – men, women, gay, straight, Protestant and Catholic, London and country. I hate that."

Scott remembers where it all began for him: “Saturday afternoon at the youth drama club, from when I was ten. We did excerpts from Shakespeare, O’Casey, Stoppard, Ibsen. I loved it. And I got from that a sense of acting being something playful. I’m still hesitant about doing too much research, I don’t want to lose that play instinct, improvising, enjoying.” He dropped out of his drama degree at Trinity College, Dublin because it was “too analytical” and got work at the city’s Abbey Theatre and the Gate, before moving to London in 1999.

Scott’s gift has taken him far on stage. Mike Bartlett’s provocatively titled play Cock opened doors, and he went on to play the elfin, camp Leo in Noël Coward’s Design for Living at the Old Vic and a fanatical ruler in the National Theatre’s Emperor and Galilean. Most recently, he was the spoilt rock star in Birdland at the Royal Court. But surely his tricksy TV Moriarty was the game-changer? At this suggestion, Scott grows animated, even a bit severe.

“I”m reluctant to say it was a game-changer because that implies you want it to change. I’ve been working 20 years – ever since ’94 – and I tell young actors to do all that, do fringe and rep, do everything. All that work, pre-profile, is incredibly important.” And how is he managing the high profile? “Well, people used to come up to me at the beginning wanting Moriarty, but I’m pleased to say they don’t so much now.”

His co-stars in Sherlock have now risen to a crazy level of fame, filling theatres with new audiences. Benedict Cumberbatch’s Hamlet has sold out a year in advance; Martin Freeman crammed the Trafalgar Studios in Richard III. In both cases there is talk of how young the audiences are – first-timers drawn by fame. So how did he feel about filling the Royal Court with people keen to gaze on Moriarty, but not too good at turning off their mobiles?

“We shouldn’t sensationalise young people going to the theatre. It is for everyone. If someone squeals briefly in excitement at recognising an actor from film or TV, that’s a small price to pay for the ongoing vibrancy of theatre. OK, perhaps a phone goes off, but that happens every night in any theatre. About Birdland, it’s true that many had never been to the Royal Court, but it’s for everybody, whether they watch Sherlock or not.” 

What is not for everybody is Scott’s private life. Civilly and gently, he fends off questions about his partners. “I am grateful I didn’t have this level of visibility at 22, because now I know that all that stuff is something you can control. I don’t have a fear of fame. But giving away even a tiny bit of your private life means it’s open to everyone. So I don’t talk. But I hate the idea of removing yourself from the human race, never getting the Tube. I don’t want people to become the enemy.” Does he do selfies with fans? “God! Yes. Well, I suppose it’s quick.”

How about a big Shakespeare lead next, like Cumberbatch and Freeman? “I am in discussions, but I would love,” he says dreamily, “to do a musical.” Can he sing and dance? “I’d need a lot of work.” He gives nothing away. But charmingly.

Pride stars Bill Nighy, Dominic West, Imelda Staunton and Andrew Scott, and is released in cinemas today

(Picture: Ian Derry/BAFTA)