The Hobbit is such a concentrated dose of imagination that it can be tricky keeping the audience on board

TURNING JRR Tolkien's novel The Hobbit into a stage show is quite a challenge. Set in a mystical world filled with goblins and elves, the story centres on a perilous quest to reclaim ancestral treasure. It's a tireless rollercoaster of a book, which finds the characters battling against one disagreeable creature after another, from angry trolls to giant spiders. Despite this, the show doesn't skimp on any of Tolkien's creations.

• Gollum (Christopher Llewellyn) threatens Bilbo (Peter Howe). Picture: Complimentary/Dan Wooller

"When we were working on the adaptation we tried to stay within the bounds of the story as much as possible," says director Roy Marsden – who, as a TV actor, is best known as PD James's police commander Adam Dalgliesh. "There's a part in the book where the characters cross the stream by boat, and we decided to use a rope swing instead – but really the majority of it is truthfully told."

Marsden's experience as an actor was useful. "I'm very aware of all the physical problems they have, because it's a very physically demanding show," he says. "And I think the actors appreciate that there is someone out there trying to be a midwife to this show, because as an actor, I know what it feels like to be up there doing it."

Described as suitable for ages eight and over, the show is aimed not just at children and families, but the vast adult following that Tolkien's work attracts – although inevitably, with a cast of 13 men and not a single woman, the audience is often heavily weighted towards one gender. "It's a very male play," says Marsden. "And 40 per cent of our audience are adult males obviously revisiting their childhood – many of whom bring their sons with them."

That may be true much of the time, but at the matinee I see it's wall-to-wall primary school children, boys and girls. "Yes," says Marsden, "the majority of them were young school kids today and yet they were absolutely silent throughout. At one point I thought, are they asleep? But actually they were just totally riveted, which is terrific."

With new characters and locations arriving every few minutes, The Hobbit demands your full attention throughout. Ensuring that both children and adult newcomers to the story don't literally lose the plot can't have been easy.

"It is difficult," concedes Marsden. "Although the scenery changes really help, because they're all indicators of place. But the real secret is to watch how the characters change. Take Bilbo for example, he's a total innocent to begin with and by the end of the play he's learnt to kill, lie and cheat – he's gone on a journey from innocence to experience."

The man responsible for showing that change is actor Peter Howe, who plays Bilbo. Having been Samwise Gamgee in a stage adaptation of Lord of the Rings, he's well versed in the complexity of Tolkien storytelling.

"There's a fine balance between making sure people understand what you're talking about, and patronising them," says Howe. "Which can be difficult, but I think audiences grasp things more easily than sometimes they're given credit for, so we try to make the story clear but not telegraph it."

From Lord of the Rings to The Hobbit, from Sam to Bilbo – what is it about this breed of story that attracts Howe?

"They're exciting to be a part of because the stakes are always so high," he says. "And the situations are all life or death. It's not 'if this doesn't work I won't get a new car' – it's 'if I don't succeed, I'll die'. So the action is always heightened. But basically it's because it's like being a kid – you get to go off and do make believe for two hours."

Yet dramas such as this can be a double-edged sword for an actor. On the one hand, if an audience buys into the incredible locations and fantastical plot, they'll go anywhere the actor takes them. If they don't, it can all feel a bit silly, as Howe is very aware.

"Audiences will only give themselves over to this kind of story, and the enormity of the situation, if it's played utterly real," he says. "If they spot a glimmer from any of the actors that they're not quite believing it, then they won't believe it either and their concentration will wane."

Like Marsden, who took on PD James's popular commander Dalgliesh, by taking on such a legendary character in literary history, Howe is carrying a lot of weight on his shoulders.

"It was the same when I was playing Samwise in Lord of the Rings," he says. "People have read The Hobbit as children, and many of them continue reading it again throughout their lives, so it has a special place in their hearts and you want to respect that. Fans of Tolkien's work are extremely passionate about it, so there is a pressure to do it justice."

That said, imagination is a very personal thing – and for every reader of The Hobbit there will be a slight variation on how Bilbo looks, talks and acts. How do you approach that as an actor?

"Everyone has a different picture in their head," says Howe. "So it's not possible to be everybody's Bilbo. But you can be the essence of everybody's Bilbo if you stick to what Tolkien wrote."

• The Hobbit is at Edinburgh Festival Theatre, 23-28 March.