Hollywood, however, is unlikely to take to the genuine Lewis. 'His friends used to call him the pork butcher,' according to A N Wilson, author of C S Lewis: A Biography. 'He was coarse, sweaty, smelly, dishevelled and drunken. He oozed nicotine - he smoked about 60 a day. And he was loud - he barked and roared.'
Clive Staples Lewis was born in Belfast in 1898. An outstanding classical scholar, he became a fellow and tutor at Magdalen College, Oxford. He converted from atheism to Christianity in 1929, and went on to become one of the greatest Christian apologists of the century - as well as a science fiction pioneer and creator of the childrens' classic Narnia series.
'He's one of the very few 20th-century Christian writers who has appealed to all types of Christian - fundamentalists, Catholics, Russian Orthodox,' says Walker. 'Most Christians find Lewis interesting even if they violently disagree with him.'
He does not like Wilson's warts-and-all portrait of Lewis any more than Attenborough's romanticised version. 'He was a person of genuine spiritual stature, but nevertheless a bit of a mess. He's had the Oprah Winfrey treatment lately - in many ways Wilson's book introduces Lewis as a real man, but diminishes him spiritually.'
Diminished or not, Lewis continues to sell. Shadowlands has taken pounds 23m in the US since its release nine weeks ago. HarperCollins publishes 27 of Lewis's religious works, plus the seven Narnia volumes. None have ever gone out of print. The most popular of his apologist books, Mere Christianity, has sold 325,000 copies since publication; The Screwtape Letters, written by a wily old devil to a novice devil instructing him on the subtleties of temptation, has sold 261,000 (sales of 100 copies are often enough to get new books onto the best-seller lists).
The Lion, the Witch and the Wardrobe, first in the Narnia series, has sold one and a quarter million copies; the Through the Wardrobe Club has around 8,000 members. HarperCollins does not even feel particularly excited by the boost that Shadowlands might provide. 'Interest never stops anyway,'said a spokeswoman.
Lewis has admirers in high places. 'The Pope is a tremendous fan,' says A N Wilson. The Archbishop of Canterbury is patron of the C S Lewis Foundation. 'The only people who won't touch him are liberal Christians like the Bishop of Oxford,' says Walker. 'They find him passe, conservative, sexist.'
He is particularly popular in America. 'He's been hijacked by the Billy Grahamists,' according to Wilson. 'At Wheaton College in Illinois they have bought up his papers, and even his wardrobe - they are hardline, fairly stupid, fundamentalists who made Lewis into a god. They see him as an intellectual who believed in all the supernatural parts of religion - who underpins all their prejudices. They see him as a dyed-in-the-wool fundamentalist, which he wasn't - he was something much stranger and subtler.'
Walker hopes that Shadowlands might boost some of Lewis's less well-known works. 'There are books within the corpus that could still fly. A Grief Observed, his diary account of his wife's death is incredibly moving.' But Lewis is already finding new markets, even without the screen boost of Shadowlands. 'He is already very big amongt New Agers.'