Most people will recognise Iain Glen from Game of Thrones, where he plays Jorah Mormont (aka Ser Friendzone), Daenerys' right-hand man and the skilled purveyor of a massive broadsword. But for Glen, his acting career isn't all about doom, gloom and medieval fantasy. This August, the Edinburgh-born actor will be hitting the big screen minus a full plate of armour, as he takes a key role in The Bad Education Movie – the cinematic spin-off from Jack Whitehall's hit BBC comedy.

The series has been compared to The Inbetweeners and it's easy to see why: both sitcoms play on the typical school yard truisms that we Brits tend find funny. Both are frank, honest, and rely heavily on the kind of comedy that is driven from strong characterisation. And crucially, both series have culminated in an 'end of school' film.

By the time the Bad Education lot have made their way on to the big screen, the term is practically over, and the feckless Mr Wickers (Jack Whitehall) is taking them on one last hurrah to Cornwall. 'He takes them on a mad adventure, where not unlike the TV series, he gets himself into all sorts of a mess. I play the mess that he gets himself into,' Glen explains.

His character is Pasco, an advocate of the Cornish Liberation Movement, which aims to achieve independence for Cornwall. Unwittingly, Wickers becomes a part of the movement, and so mayhem ensues.

Surprisingly, Glen did very little research about the area before filming took place, as 'any research it would have required was within the lines.' 'I was a bit lazy with that,' he admits, but when it came to nailing the character, he certainly did his homework.

'My only research was to try and nail the dialect,' he says. 'I always enjoy dialects and it was a fun one to do, but in terms of the history of Cornwall or the particular movement, l hold my hands up and say it was all there in the script.'

It wasn't just a history lesson that Glen gained from the writing. Before being offered the role, he hadn't seen the Bad Education television series, but after reading Whitehall's work, he soon rectified that. 'I've got kids at the wrong age, or I'm at the wrong age, and so I missed it when it came out', he explains. 'I immediately watched it when they offered [the part] and I just really, really loved it. I thought it was very funny and I think Jack's a bit of a genius.'

His relationship with Whitehall blossomed while on set, and the chemistry between them helped to inform the comedy. 'He's a very lovely guy, and he was great fun to hang out with. We became good buddies.

'Comedically, we have very different styles. He has a more traditional style and I have a more spontaneous style, less concerned with the form,' he says. But Glen feels that this added to their scenes together, particularly one in which they sword fight (something he's already experienced on Game of Thrones).

'Since I trained at the Royal Academy of Dramatic Art in London, I've always loved sword fighting and I'm very keen to grab any opportunity to use it,' he says. 'I hope I was quite helpful in the scene because it's probably more familiar territory to me than it was to Jack.'

Sword clashing may be familiar territory, but the whole idea of an outrageous class outing is alien him. 'I don't think I went on a single school trip,' he says, recalling his own days in full time education. 'It was a different era. You have to remember that I'm very, very old and I had a very different style of education. I'm sure there were the odd school trips somewhere, but I didn't go on any of them. I was probably just wanting to hang out with my mates and get away from the school.'

In fact, Glen admits to being an 'irritant' in the classroom. 'I don't think I was very good academically,' he says. 'I didn't really respond to the subjects that were being taught to me, so I would say I was a bit of a rebel, and somehow scrambled into university. I don't quite know how I did it. I would say that of the classroom, I would be the one of them chipping from the sides, just trying to undermine the teacher.'

Despite his own lack of reference points, and the comedic style being intrinsically British, will this film appeal to the universal masses? 'I hope so,' he says. 'It's a funny thing, but often if you're very true to the world that you're portraying and you really localise it and fill it full of local detail, then curiously it becomes universal.' And with that attitude, Glen, Whitehall and co are all set to school the big screen in the true meaning of Bad Education.

The Bad Education Movie is on general release from Fri 21 Aug.