Spring is here. And that can mean only one thing: winter is coming. Last year, unsuspecting viewers were first introduced to the bloodied world of Westeros. If you've yet to watch, Game Of Thrones is a fantasy following the machinations of nine long-standing dynasties spread across seven kingdoms, all fighting to take control of the seat of power – the Iron Throne. There's incest, infanticide and adultery – but also starvation, corruption and council meetings. We meet giant wolves in the forests, burly knights in brothels and tiny kids in battle; kings who have no interest in ruling, and politicians who can barely contain their contempt for authority. You can lose your head (literally) if you cross the wrong man, seasons last for years (that's why no one wants summer to end), and the civilised court is one of the most savage arenas. The cast of the programme is populated by a strong British and Irish contingent – Lena Headey, Charles Dance, Iain Glen, Michelle Fairley, Aidan Gillen, Roger Allam, Jerome ("not Robson") Flynn: all decent actors, but few have star power to outshine their character. And they're all out-acted by Peter Dinklage, whose "Imp" Tyrion Lannister runs away with every scene he's in. As for it being a fantasy – probably the only genre afforded less critical respect by mainstream critics than sci-fi – the wilder elements are there, but carefully rationed. We had dragons' eggs, but no dragons until the final moments of last year's series.

Scene-stealer Tyrion

"The Sopranos with swords" is how it was initially pitched when HBO announced its first fantasy show. That's TV shorthand for "yes it's a fantasy, but it's a grown-up one and that includes death. And probably nudity." Downton Abbey's Iain Glen offers an alternative comparison: the genteel Granthams and Westeros's barbarian Dothraki tribe. "The central things that humans do to each other remain the same. In Downton, Maggie Smith might do it with a particularly well-chosen phrase; in Game Of Thrones they might be stabbing you with a sword."

The series is an adaptation of A Song Of Fire And Ice, a series of doorstop fantasy novels by George RR Martin that's sold over 15m copies worldwide. In just a year since Game Of Thrones first aired, the show has built on the novel's audience to become a global hit, watched everywhere from Argentina to Vietnam. In the US, first-week box set sales outstripped figures for previous HBO big-hitters The Sopranos, The Wire and even Sex And The City. In the UK it's been at the top of Amazon's TV chart for over 100 days. The opening sequence – an articulated map of the world that looks like it's been constructed by Swiss clockmakers – has already been spoofed by The Simpsons – a real "you know you've made it when … " pop culture moment. Even the Iron Throne itself has been on tour, with fans queuing to see it and the rest of the meticulously crafted props.

If it doesn't seem like a surprise, it's worth remembering that the last big fantasy series on TV to have any kind of international impact was probably Xena: Warrior Princess – pulpy teatime fun, but nothing to worry the Emmys. Game Of Thrones meanwhile received 13 nominations last year. The Guide met the cast and writers David Benioff and DB Weiss in London to talk about the show and its immediate success. Benioff and Weiss are both experienced writers who've worked on everything from Troy to The Kite Runner; they're an understated, but focused pair who've been friends since studying Irish literature at Trinity College, Dublin. "Dan was writing about Joyce, I was writing about Beckett," says Benioff, "now we're writing about dragons." Both freely admit to being high-school Dungeon Masters; you can tell they're relishing the chance to work on something they'd have loved as teens.

Daenerys

Weiss explains how Martin's world drew them in: "One of the things we love about this story is how it lets the unreal and the fantastic bleed into the very real and psychologically complex human reality of the show in doses; you're not just getting buckets of magic dumped on your head from square one and being asked to accept these people's lives as real even though crazy, fantastic stuff is happening. It happens in pieces: the dragons get bigger, and the wolves start out as more doglike, because they were dogs, and now they're not dogs any more, they're wolves the size of small ponies … There's an acclimatisation process that's going to keep people with the story even as some more fantastic elements come into play, which we're really excited about because it is a fantasy show. I don't think it transcends its genre; I think we hope for it to be a really, really great example; we wouldn't be working in the genre if we didn't really love it."

For such a colourful story, what's notable is just how many shades of grey there are. Simple binary oppositions like good and evil become altogether murkier propositions. The show turns so that obvious villains like sister-brother lovers Cersei and Jamie Lannister have moments where you suddenly see the world from their point of view (as the actor who plays Jamie, Nikolaj Coster-Waldau, puts it, for an "incestuous child killer" Jamie is "very honest and witty – you can't just hate him"). Their drunkard, whoring, "disappointing brother" Tyrion (Peter Dinklage) turns out to be one of the most consummate players on Westeros. Meanwhile, it transpires that a noble family man like Sean Bean's Ned Stark has fathered a bastard son, Jon Snow, who Ned then expects wife Catelyn (Michelle Fairley) to raise; effectively asking her "to love someone who's been completely dishonourable", as Fairley puts it with a wry smile.

In adapting a highly successful series of novels, the creators didn't have to just satisfy themselves. Benioff tells a story about the first screening for HBO and Martin – the first time he'd seen any of it. Martin's wife Parris turned around and looked at them with this half-joke, half-threat: "Don't fuck it up." After the second episode they were relieved when she turned around again: "You didn't fuck it up."

Jon Snow

To get the job, they first had lunch with Martin, who quizzed them on one of the novels' biggest mysteries: who do you think Jon Snow's mother is? "Luckily we got it right," laughs Benioff. But Martin – a former TV writer himself – quickly followed his approval with another proviso. "I've always said I wanted this to be an unproducable show," he told them. "I always wanted to make it bigger than anyone could possibly make it, and not have to worry about any of those realities – and now you guys are going to have to worry about it."

Whenever you talk to actors about work they're promoting you expect a degree of platitudes and "best experience ever" stories. That said, there's a sense of commitment – and how much they're enjoying the end result – as the cast talk about their characters, whether it's Emilia Clarke, fresh from drama school, laughing as she describes how "Danny" helped her through her first nude scenes with man mountain Jason Momoa; or Liam Cunningham, who watched the first season as a fan and then couldn't believe his luck at joining the cast to play "the Onion Knight" Davos. Even seasoned pro Charles Dance switches from tales of sharing an agent with Sir John Gielgud and roles he took in the past just for the location ("Six weeks in Beirut, lovely city, terrible film") to admitting that he's been poring through the scripts to find out what the other characters are saying behind Tywin's back, and raving about the quality of lines he gets to deliver as the battle-weary head of the richest clan in Westeros. "The public are behind Arya Stark, a little girl who would put a knife in my back if she had the chance," he says. "But what the writers are being very clever about is denying her the physical opportunity, and also writing scenes that allow Tywin Lannister to give you a little glimpse of what's going on inside [him] – which makes an audience say to the little girl, 'Don't kill him yet – there's something more we want to know!'"

Game Of Thrones, Mon, 9pm, Sky Atlantic