WHEN Kit Harington was cast as a spy in Spooks: The Greater Good, the new movie version of the hit BBC drama, it was seen as an advantage that he had not watched the show.

His character had to come across as a true outsider, an unknown quantity. What the producers did not know is that the Game of Thrones star had done some real life undercover work in his career - in Glasgow, no less.

It was a few years back and Harington, now 27, was still a student at the Central School of Speech and Drama in London.

"I had to do a Glaswegian accent so I went up to Glasgow to try to get the accent down."

The "op", carried out by Harington and his then girlfriend, involved a visit to the Snaffle Bit pub in Sauchiehall Street.

"We both tried to stay in the accent all the way through. We were probably awful at it but it did help going and seeping up the feel of the place."

While in the city he also took "the rainiest, coldest bus tour I've ever done". Yes, he sat up top, in winter. It was more good training, this time for the rigours of filming Game of Thrones in Iceland and Northern Ireland. Save for a trip to Berlin, Spooks: The Greater Good was a more home grown affair with much of the filming done in Harington's home town of London. Setting the tone of the piece is Peter Firth who returns as Harry Pearce, the MI5 spymaster crumpled by grief and weary from experience. Harington's character, Will Holloway, represents the new school of espionage, one in which high tech plays as much a part as low cunning.

Holloway sports a distinctly cool, un-Harry like hair for a spy. It is a barnet, indeed, not a million cuts away from that sported by Harington's character in Thrones, Jon Snow. There is a reason for that. While making Thrones, Harington is contractually obliged to keep a shaggy do. So no dye, no cutting, nothing.

The HBO fantasy drama, now in its fifth season, really is that big a deal. Based on A Song of Fire and Ice, the mega-selling novels of George RR Martin, the multi-award winning tale of sex, war, skullduggery and dragons has done for fantasy what The Sopranos did for interest in the mafia. Harington believes his character became popular with fans because Snow is a beacon of hope in a bleak world. "There are a lot of very awful people in Game of Thrones. There's a little redemption for the story in Jon Snow, who is actually a good person, a nice person. He's quite dour but there is hope with him, there's hope for the story."

As for the success of the show as a whole, it's all about the grime, Harington reckons. Previously, fantasy had been too squeaky clean, too glossy. "What Thrones did, which hadn't been done before, was put mud on people's faces, make them grimy and real. That was a masterstroke, people responded to that."

Though best known today for Thrones, and the films Pompeii, Seventh Son, and the acclaimed Testament of Youth, Harington began his career in the theatre, playing Albert, the young First World War soldier in War Horse. I confess to him that having sobbed my way through the Michael Morpugo novel and the Spielberg film, I did not think I could manage the play.

"My heart goes out to whoever plays that part. It's a very heavy, emotional, tumultuous journey that actor has to take every evening, along with the guys who play the horse. It is harrowing, but it is also beautiful. I have very fond memories of it but yes, each night you had to take yourself to a place of utter desperation, and you had to grow up each evening."

After War Horse he appeared in another stage hit, Posh. He misses the theatre. "It's a balance like anything is, and I want to be able to balance doing both. At the moment I've been away from theatre for so long I would feel presumptuous and arrogant to think I could waltz back into it." He will return though. "I don't want a singular TV/movie career, I want both. I want it all," he says, laughing. Does that include writing and directing?

"I'd like to write and produce. I want to create things and help things get made if possible but you have to have a certain mind for directing and I'm not sure whether I have that mind. You have to be absolutely 100 per cent sure of what you are doing, you can't have that usual actor thing of doubting because you are dealing with a whole load of actors who are doubting themselves constantly. You have to be the rock and I don't know if I am that rock."

Years before becoming Jon Snow on screen, Harington's only experience of the Fire and Ice novels was hauling them into the bookshop where he then worked. "At that point I had no idea what they were other than that they were very heavy. I wasn't his biggest fan. Now I couldn't thank him enough."

Thrones has made him financially stable, for which he will always be grateful. "But that's not the first thing. I've been given an opportunity to be part of something which is very special, which doesn't come along very often, and which has affected a lot of people. People have really enjoyed and it has become part of their life, it has entered into the zeitgeist."

The only downside is that it can become all-consuming, he adds. "It becomes so much part of your identity, who you are, and your life, that you have to constantly remind yourself in your private life that that's not the be all and end all of everything and there are other things in the world than Game of Thrones. That is sometimes tougher than you might think."

Coming up is a holiday in June, and talks with the French director Xavier Dolan about a possible new film, The Death and Life of John F Donovan. Later this year, Harington will be seen as a tennis player in the HBO mockumentary, 7 Days in Hell, about a marathon tennis match.

Though Harington is a long time Murray fan - "He's a fantastic tennis player, a sportsman that we've been wanting to have for years and years in the UK" - he had never played the game. He soon realised what a disadvantage this was when the cameras rolled on 7 Days.

"I was absolutely awful. I don't have the stature for a tennis player. You've got to be quite tall, at least 6 foot. I'm almost shorter than the net."

Before he goes, back to Spooks. Should another spy role come up, something to do with the code 007, for example, would he be up for it?

"It's not something I think is going to offer itself up [but] if I got the call to play Bond I think I'd consider it, put it that way."

[itals] Spooks: The Greater Good is in cinemas now