So that's how the daemons work. But what about the bears?

Suzi Feay meets the young actors bringing Lyra and Will to life in the National Theatre version of Philip Pullman's 'His Dark Materials'

Questions flow forth from the Philip Pullman fan, on learning that the magisterial children's fantasy trilogy His Dark Materials is being adapted for the National Theatre as its Christmas show. Like: how on earth are they going to do the warrior bears? The cliff-ghasts? Are there really going to be hot-air balloons and flying witches? What about the land of the dead? How, for heaven's sake, are they going to represent the daemons?

I would really like to answer those questions, or at least give some sort of hint about how this richly imagined world is going to be created for the stage. Unfortunately, all I've got to go on is one shoe. A Converse baseball sneaker, to be precise, on the foot of Dominic Cooper, who plays Will, the boy who meets Lyra, a young girl who lives in a parallel universe, and, after many adventures, falls in love with her.

Such is the secrecy surrounding the production that no part of the set or piece of costume can yet be revealed. Apart from that shoe. Cooper and his co-star, Anna Maxwell Martin, are larking about in the foyer of the National Theatre. They cut in, finish each other's sentences and tease one another. Fortunately, they're also a bit more forthcoming on the details.

"I'm very jealous of all her nice costumes. You've got lovely big fur interesting eskimo jackets..."

Anna: "He's really jealous that I've had a coat made."

Dominic: "...and I'm wearing Gap best."

John Morrell, the costume designer confirms later: "Dom's drawn the short straw. He's a boy from our world, and he's just got to look like a regular kid in the middle of all this fantasy and quirkiness. Dominic is innately kind of hip and trendy, and we've had to try and knock that down. He wears clothes a little bit too well, really."

Back to the daemons: for the uninitiated, every human character in Lyra's world has a personal daemon, or familiar, in the shape of an animal. Anyone who's ever read the books will know that this is one of the most thrilling, poignant and original aspects of the story. Some of the daemons are characters in their own right. But how can this device, so brilliant on the page, ever be made to work on stage? Anna explains: "The demons are puppets, so we have puppeteers but they're also manipulated by the actors. For Lyra, Pan [Pantalaimon, her daemon] is such an important character and also Pan talks a lot, so there's a puppeteer who is Pan and does the voice, and I just take him sometimes..." She plays with an imaginary puppet in her lap. "You have to make sure you don't forget and just do THIS!", abruptly pointing and gesturing with her puppet hand. "Your brain is constantly split between doing your lines and remembering to give life to this little thing."

Dominic: "If anyone stops for a moment and forgets, it really shows."

The story of two young people exploring a fantastical series of words is mirrored in the pair's evident delight in discovering the resources of the mighty Olivier theatre.

Dominic says delightedly: "There's this huge droom in the middle, this operating thing that goes round and up and down, and it's had another section built on top of it. I haven't even been in there yet. And that hasn't been used for years, has it?"

What's it called, I ask?

Anna picks up: "It's called a droom, it's a room under the stage. It's so exciting, isn't it?" She gasps: "We shouldn't give that away, it's going to be a real shocker when it comes up."

Cooper deadpans: "There is no droom."

One of the most powerful scenes in the book is a relatively quiet one, right towards the end. We've been building up some time to the notion that Will and Lyra are in fact the new Adam and Eve. What's that like?

Anna: "People have asked us, how much do Will and Lyra age, and it's not clear... they certainly mature. I think she's about 12 to begin with, but it's important to get to a point where it would be feasible that we would fall in love. And make love, you know?"

Dominic: "You get that feeling of the journey they've been on to reach that point where they could, and where they would... But we're doing sex workshops next week."

Anna screams with laughter. "Please put that in. '12-year-old Will and Lyra have a sex workshop.' What, just us and Nicholas Hytner? Oh God! You can do it on your own, love."

Pullman's vast drama is not exactly comforting Christmas fare and both actors are confident that certain scenes will chill audience members to the marrow. And the ending is absolutely harrowing. (Anna: "I cried my eyes out when I read the book.") Dominic muses: "I so would have kept the subtle knife. I don't know why they didn't..."

Anna: "We would never have got into that situation to begin with."

Dominic: "We would never have gone through the window."

Anna: "We are wimps!"

'His Dark Materials': National Theatre, Olivier (020 7452 3000), to 20 March