THOUGH the last episode of Sherlock hardly wanted for surprises, there was a coast to coast intake of breath when the dastardly Moriarty, previously seen committing suicide, popped up in the final frames to ask:

"Did you miss me?"

There is no chance of missing Andrew Scott, the actor who plays the Napoleon of crime in the BBC1 series, because he is about to appear in a new comedy film The Stag, followed by Locke, with Tom Hardy, Jimmy's Hall, directed by Ken Loach, miners' strike drama Pride, and then a new version of Frankenstein, with James McAvoy and Daniel Radcliffe.

One question that requires an answer before we talk about The Stag is whether Scott, 37, was one of those who managed to figure out how Sherlock, never mind Moriarty, came back from the dead.

"I worked it out, but I worked it out having read the script," he laughs. Elementary, really.

The Stag is a universe away from Baker Street. Directed and co-written by John Butler, it stars Scott as Davin, best man to his best friend.

The groom doesn't want a stag weekend, Davin doesn't want to organise one, but the bride thinks it would be a grand idea, especially if they take along her macho brother. Tears, and a lot more besides, occur before bedtime.

All concerned with the project are united on one thing: it is not the Celtic Hangover. While it is a comedy, there is real heart besides, he says.

"It's in no way like the Irish version of The Hangover, which was the concern for everybody. We wanted to make something that was as appealing for men as it was for women. We didn't want to alienate anybody."

In the film, the friends go on a walking weekend in the Dublin mountains.

The idea of heading off to a European city and drinking as much as possible, "organised fun in a wilful sort of way", is not his idea of a good time.

"I don't think for a lot of guys it is; they might pretend it is. I think sometimes those things are just to cover up massive anxiety."

Since premiering in Toronto, Dublin, and most recently Glasgow, the film has become a word of mouth hit. "People were clapping after seeing it. Just the idea of 700 people in a cinema really laughing and enjoying it, having a little cry, and just feeling good. To my mind comedy is underrated."

Among those who will be booking tickets are his Sherlock fans. Just as Benedict Cumberbatch has his self-proclaimed "Cumberbitches", Scott has his Scotties. Being young, Irish, slight, and not that well-known outside the theatre, Scott acknowledges he was audacious casting for a part once played by Orson Welles and Vincent Price.

"I don't think one person would have gone, 'Do you know who the man for that job is ... Andrew Scott," he laughs.

Yet, judging by the response, series creators Steven Moffat and Mark Gatiss had found their perfect villain. His first appearance in 2010 sent Twitter doolally, and he went on to win a best supporting actor Bafta in 2012.

Born and brought up in Dublin with his two sisters (his mother was a teacher, his father a manager), Scott filmed his first movie, Korea, when he was 17. Other film parts followed, notably Saving Private Ryan, but his career pre-Sherlock was mainly in the theatre.

From early roles at the Abbey Theatre in Dublin after dropping out of his drama degree at Trinity, he moved to London, appearing in Dublin Carol with Brian Cox, among many other productions. Olivier Awards followed. Then came Broadway and The Vertical Hour, alongside Julianne Moore and Bill Nighy. In the meantime, the television parts, including playing Paul McCartney in Lennon Naked, kept coming.

But it is Sherlock, sold around the world, that has brought him the kind of fame that means he is now recognised in the street. It is something he is still getting used to.

"I've had the luxury of working as an actor for 20 years and I want to protect that, and protect my life. But for the most part people are very nice.

"I like to deal with the public in the way that I've always dealt with them. If people are pleasant I love to have a chat with [them], that is maybe an Irish thing, and I'll always try to take a photograph with someone if I'm not working or with somebody. But if people are rude I don't like it.

"For the most part people are okay, you just have to try to have balance in all things."

When we speak he is in rehearsals for a new play, Birdland, which opens at the Royal Court in London on April 3. Strangely enough it is about a rock star, played by Scott, who is grappling with fame.

"It is the actors' medium, it really is," he says when asked if theatre was the ideal training ground. "You are in control of everything, you have to tell the story, you are the chief storyteller. They can't add in incidental sad music to make the audience feel more about you, you can't edit it in such a way that the audience are going to laugh or whatever. You are in charge of that, absolutely. That live experience, that responsibility you have as a theatre actor can only increase your confidence in a sense."

It also helps with dealing with popularity if it comes, he reckons. "I do a little bit of mentoring with young actors and I'm always saying 'don't try to rush to fame too much' because I feel very grateful that I didn't have too much scrutiny too early on. So when scrutiny does come, when people are pushing you in all sorts of directions, you can say 'well no, I know I want to do this, I know I'm good at that'."

He found a fellow board treader in Glasgow's James McAvoy, recently seen on stage in Macbeth. In Frankenstein, McAvoy plays the titular doctor of science, with Scott as the man of faith who challenges him.

"He's just had the most extraordinary career," says Scott of McAvoy. "I like the way he is all about the work and he's great fun."

The next Sherlock is on the way, but will not appear till 2016. Scott's lips are superglued about what it might contain, largely because they haven't set a date to start filming yet. "Whenever we can get everybody in the same country, city, continent, we'll do it," he promises.

Missing him already.

The Stag is in cinemas now