"I would have been very, very uneasy if John had been disturbed by the script or if he had felt in any way it traduced his memories of Iris or his own behaviour…"

The screenplay for the film Iris, based on John Bayley's bestselling memoir, was adapted via email by director Richard Eyre and writer Charles Wood over a period of six months.

Shortly before the death of author and philosopher Iris Murdoch in 1999, her husband, author and academic John Bayley, wrote Iris: A Memoir (published as Elegy For Iris in the United States), a frank, moving and sometimes humorous account of his life with the woman who was frequently described as "the most brilliant woman in England." The latter part of the book dealt poignantly with the effect of Alzheimer's on Iris as well as John's selfless devotion to his wife of 43 years.

"Essentially, Iris is about forms of love and the way in which love changes and love endures," says director Richard Eyre, who co-wrote the screenplay with Charles Wood. "Iris is first and foremost a love story and I make no bones about that. It is a story of enduring love, a story about love and old age which covers Iris's whole life. In a sense it reflects on everyone, because in every relationship you have to accommodate the otherness of the other person and that's very much what it's about. It also explores how you can be separate beings in a marriage and yet the sum of the marriage is greater than the parts."

Richard Eyre's mother suffered from Alzheimer's - an experience which he described in his autobiography, Utopia And Other Places. "The particular agony of Alzheimer's is that it robs a person of their being and of their personality," explains Eyre. "Although in some ways they remain who they are, somehow they are constantly diminished and you just see the person they once were gradually disappear. It's agonising. One of the things that I've tried to show in the film is that even though the person is disappearing in front of you, in some way there is a sense in which they remain. You can still love the person because their soul is still there until the end."

When it came to writing the screenplay Eyre turned to Charles Wood, with whom he had collaborated on the BBC television drama Tumbledown, about the 1982 Falklands War. "We started with the premise that it had to be a double narrative," explains Eyre. "The idea of someone losing their memory and losing the faculty of language was a very potent theme, and that was the spine of the story. The tension throughout the film was always to be driven by the youth of the young couple and the decay of the old couple - the young couple falling in love, and the old couple staying in love - and the two stories converging."

"I'd wanted to work with Richard again, and I was delighted to be asked to work on Iris because it is a cracking good story," says Charles Wood. "It's the first time I've co-written with someone else but I will do it again with alacrity. Writing with Richard was the most marvellous experience. It was surprisingly painless."

Every evening for a period of six months, Eyre and Wood would email each other the full script, showing every change. "To start with I wrote a rough screenplay, and then Richard started working on that," says Wood. "He would re-write a scene or change some of the structure, then he'd send it back to me. I'd have a go at it and show it back to him. Then he'd email it back and I'd have another go at it. It was total collaboration at all times."

Deciding on a structure for the film was an early source of difficulty. "One of the most difficult things about the writing was how to make a film that didn't immediately plunge you into the misery of Iris's illness," says Eyre. "In the final film, you don't see the illness for quite a long time. That's because we use the device of seeing John and Iris as young people, gloriously unaware of their fate. The strategy is to ambush the audience and to surprise the audience when you're in one reality and you go back to another reality. It's quite a simple structure. You're not dealing with more tenses than past and present."

Charles Wood agrees. "Getting the tone of the film right was probably the hardest part," he says. "It mustn't be overly sentimental. It must be a convincing picture of people's lives." Wood had met Iris Murdoch twice at parties in Oxford and found the fact useful when it came to writing dialogue. "She was marvellous, very jolly and very interesting but in the most ordinary sense - there was no nonsense about being the great novelist. This was a woman who was totally at ease with herself and her position. Both she and John were open and friendly. It's always difficult to put words into the mouths of people who have existed if you haven't ever met them," he says. "But having met her I felt quite at home giving her words to say."

Deciding where to make changes to John's Bayley's books for dramatic reasons was a fundamental decision for Eyre and Wood. "I guess at quite an early stage we stopped feeling doggedly faithful to documentary reality," Eyre says. "But I don't feel we've been in any way untruthful to Iris Murdoch's experience - nor did John Bayley when he read the script. I would have been very, very uneasy if John had been disturbed by the script or if he had felt in any way it traduced his memories of Iris or his own behaviour. Getting his blessing was important to me and he did give his wholesale blessing and said he thought that Iris would have approved. That was a good moment."

For his part, Richard Eyre recalls an occasion when he met Iris Murdoch. "I met her in the summer of 1997 when I was a visiting lecturer at St Catherine's College, Oxford, which was John Bayley's college. There was a cocktail party and John was there with Iris and he introduced me. She said to me: 'And what do you do?' and I told her; and then a couple of minutes later she said: 'And what do you do?'"

director richard eyre talks to author john bayley