After the massive cinematic success of Peter Jackson’s version of the Lord of the Rings trilogy, it was almost inevitable that Hollywood would turn its attention to Narnia. Another magical world, another battle between good and evil. But where Tolkien’s Middle Earth trilogy offers an explicitly epic scenario, the books in C S Lewis’s seven-part series, each set in a separate time, place and literary genre, are slender volumes, redolent with whispers, scents and feelings. Dramatic events are often merely sketched in as backdrop to the real action, which occurs inside the heads and hearts of characters choosing between power and service, trust and scorn, betrayal and self-abnegation.

There is something very fleeting about Lewis’s Narnia, something which cannot quite be pinned down. It is partly the allegory - the deeper meaning of the story’s sacrifice, redemption and resurrection is only ever hinted at - partly the fluctuating nature of its protagonists, one moment squabbling children, the next traitors and heroes.

All of the child stars in Andrew Adamson’s new film of The Lion, the Witch and the Wardrobe look exactly right for this non-idealised world: innocent, slightly clunky, all grey socks and homespun haircuts. Several are also remarkably good actors: Skandar Keynes simmers real anger beneath the sullenness of Edmund, while 10-year-old Georgie Henley, scooped with no acting experience from her Yorkshire primary school, is a vivid and natural Lucy.

In fact, the whole film looks magnificent. Both Tilda Swinton, coruscatingly pallid as the White Witch, and James McAvoy, rosily bacchanalian as the faun Tumnus, could have stepped straight from the Pauline Baynes illustrations which have become synonymous with the books.

Their homes - turreted castle, bibliophile’s cave (Lewis enthusiasts even get a glimpse of the authentic jokey academic book spines) - are lovingly recreated, if on a more grandiose scale. The animation is stunning: minotaurs, wolves, hideous hags, awe-inspiring centaurs. There is as much glittering frost, snow and ice as anyone could ask for.

It seems almost ungracious, in that case, to suggest that somehow the film misses Lewis’s point. But it does. From its fortissimo opening, all air raids and panic and tearful evacuation scenes, it is obvious that Adamson has decided to broaden out Lewis’s story. By doing so he makes both the human and the fantasy world very real. We are thrown into them, and - seated together in the cinema - we are all thrown in together. Narnia becomes a shared, corporate experience. A very striking one, but very different from that created by Lewis, the setter off of hares in the reader’s imagination.

Frankly, having committed himself to making the film, Adamson had little choice. Lewis implies so much in so few words. In the book the children are despatched to the country simply “because of the air raids”. No back story there. The film’s magic wardrobe, gateway to Narnia, towers hand-carved, alluringly draped, glimmering with Disneyfied promise. In the book it is simply “a big wardrobe, the sort that has a looking glass in the door”.

Adamson’s battle is the film’s tour de force: a quarter of an hour of bloody charges and flashing blades. Lewis’s battle lasts two paragraphs.

The rest is in our heads.

For anyone who likes watching battles, the film’s state-of-the-art one will stir them to the edge of their seats. But the tension around the book’s battle is not due to the adrenalin rush of vicarious aggression, but to the uncertainty over who will win. And yet, in the film, the winning side is never in doubt. From the moment Liam Neeson’s smug voice comes over the sound system, sounding as though he is reading the part of Aslan, Narnia’s leonine overlord, from the depths of a leather sofa, we know that this Lion-King is going to come out on top. Worse than that, Adamson back-pedals on the reason why Aslan has to die: that he has to restore the havoc caused by traitorous Edmund. The resurrection scene in the film is rather a non-event. The moment in the book when Edmund’s two sisters wonder whether Aslan will tell their brother just what he has endured for his sake never appears in the film. The baddie is the White Witch, not a human being.

Things are simpler that way.

For Narnia devotees, this is probably too much to tolerate. For people who see the film then buy the book, expecting the same high colour, epic drama and family values, there may be a surprise in store. But for teachers of English and film studies there is a an exciting opportunity - backed up by some good web-based ideas for creating storyboards, scenarios, music, set and costume design - for thinking about just what it is that books can do, and can’t do, and that films can do, and can’t do. And the gains and losses in both.

* The Lion, the Witch and the Wardrobe is in cinemas from this week. The seven Narnia novels are published by HarperCollins. Ideas for key stage 23 activities related to the book and the film can be found at www.filmeducation.org and www.collinseducation.com