If today's sense of whimsical, tide-watching abandon comes as a relief for Miller, it's only because he's spent the past three years examining what happens when the eyes are, instead, privy to something profoundly terrible.

The Optimists explores the effects of trauma - not just on the mind and soul, but on the whole body. On the surface it appears to be about an emotionally disconnected family, yet behind that looms a brooding malignance - essentially, the effect of witnessing man at his most brutal.

Clem Glass is a photographer sent to document a genocidal massacre in a African country referred to in the text as 'N'. By Miller's own admission this is a thinly disguised Rwanda - to the point that, in the uncorrected proofs sent out last year, it's accidentally referred to as 'R'. Glass returns from his assignment a broken man, trapped in an emotional vacuum and unable to comprehend, let alone countenance, the scale of the atrocity or how it has affected him.

Miller's responsibility in attending to Glass's malaise, therefore, was to become immersed in it, too. Bright, cheery and eloquent over the phone he may be, but he has had to dig himself out of a dark place to be here.

'Because of the nature of the material that I was spending a lot of time with, researching and filling my head with second-hand images - god knows how you deal with it first hand - I found that it began to subtly affect my mood,' he says.

'It darkens you over that period of time. It gets inside you, so I became a little closer to Clem than I wanted. Maybe that's what had to happen. I found by the end of writing the book that the thing had left a kind of silt or sediment in me.'

Glass questions whether optimism can ever be more than self-deceit, and takes a physical and metaphysical journey to find the answer. He leaves London for the remote island where his father lives in a brotherhood of silence; to a Scottish clinic where his sister, Clare, is recovering from a nervous breakdown; to Toronto with his old journo colleague Silverman; and to Brussels, where the war criminal Ruzindana is apparently hiding.

Such freedom of movement is typical of Miller. His novels bounce across nations with abandon. Ingenious Pain (1998), the tale of an 18th-century surgeon unable to feel emotional or physical suffering, takes in England, Europe and Russia. Casanova (1999) throws the Venetian seducer into the bowels of London. Oxygen sweeps through the US, England and Eastern Europe.

The winner of the James Tait Black Memorial Prize and a former Booker short-listed author, Miller's life clearly informs such geographic ambition. He has worked in a hospital for the mentally ill, at a chicken abattoir and at a gherkin-pickling factory in the Netherlands.

Obtaining a first-class degree in literature, philosophy and history in 1985, he married that same year and divorced five years later. Gaining a masters in creative writing, he then taught English in Spain and Tokyo before returning home to write his debut novel. Now on to his fourth, the process of having the past few years of your life chewed over by critics is as unpleasant as ever for Miller. 'You feel that you'd rather be somewhere a long, long way from where this is happening,' he says. 'You feel that this book is a vulnerable little creature, slightly patched. There are things that aren't right with it, and yet out into the world it goes to be put under cruel scrutiny by people. Which is unnerving and there's nothing I can do to protect it. And it tends to feel like a judgment - not just on the book but on the last three years of your life, and I find it hard to be completely indifferent to that. So for safety's sake I keep myself a certain distance from the review.'

The thread of continuity that stretches across his work is that of people caught in a state of emotional numbness. Miller's ability to nail sentiments of humanity and morality is unique in its exactness, and has won him praise.

'There's a continuation in theme of the problem of feeling - James Dyer in Ingenious Pain doesn't feel anything,' says Miller. 'Casanova perhaps doesn't feel as he thinks he did - he can't get the same kind of enthusiasm for the things that spurred him on in his youth. Alec in Oxygen is fearful that he can't show his mother what he's feeling - that he has no emotional vocabulary to express what he's feeling for her. And then for Clem in The Optimists, the machinery of feeling has been over-wound, like a system that's had some kind of a power charge and just shuts down.'

The horrors Clem revisits are informed by the experiences of British war reporter Fergal Keane, whose account of his time in Rwanda prefaces part one of the book. 'We had learned something about the soul of man that would leave us with nightmares long into the future,' he writes.

In conveying Glass' trauma, Miller explores the notion that the eyes themselves can be affected by what they see. Glass sees everything through the cold, mechanical eye of his camera, and yet he notices the effect of such brutal visions on his own eyes: 'With his left hand he covered his left eye. The right, singled out, looked back at him like a small cowed animal. Could an eye be damaged by what it looked at? Too fierce a light will burn it. What else? The eye was not a machine. Could it look on a living child the way it looked on a child's two-day corpse? Hard to believe something so delicate would not be altered.'

With Asia still reeling from the December tsunami and the earthquake's aftershocks, the release of a novel dissecting the issue of trauma and loss of faith seems eerily prescient. And suddenly it seems as if the dam has broken on the subject of Rwanda too, with a slew of interpretations coming more than 10 years after the event, such as last year's film Hotel Rwanda.

'I hadn't anticipated this happening at all,' says Miller. 'It's odd because these things are surfacing much at the same time. In terms of the experience of the writer or artist, what has just happened isn't available to you immediately. There's always a period of time in which that material is somehow processed. A lot is forgotten, a lot is remembered, and things have to be put in shape.

'I guess it depends on the event. People were writing books and making films about the second world war pretty much immediately after it stopped, but then there wasn't a problem as to what happened being fairly straightforward, perhaps. Rwanda was perhaps a difficult thing for us to admit - partly how we failed to help in what was a complicated issue, with roots going back into colonialism. I've been taken by surprise by the fact that there is this kind of raft of things now. I thought Hotel Rwanda was fantastic. In all these situations, however bleak and however many people are behaving in monstrous fashion, there is always somebody who isn't. That's at least as interesting as the crimes or the violence that have been carried out.'

The events in Sudan are a grisly mirror of what happened more than a decade ago, and Miller is fortunate that he only has to escape the darkest recesses of his own imagination. One of the many secrets to his writing, he says, is paying attention to the small things in life. Making the micro macro.

'A writer needs to attend,' he says. 'Attend to the details of people's lives and to the little stuff of the world. As much as anything when I'm writing, I enjoy attending to the small. I'm very keen that writing should be very visual and very physical. It's very important to me and I'm kind of intrigued by the ordinary stuff, the stuff that we kind of just pass by - say, when we get on a plane - which we don't attend to. As a writer you rescue those details from the oblivion that familiarity condemns them to.'

It was another unexpected small thing, however, that eventually enabled him to pull himself out of the funk that haunts his character Clem.

'I was rescued by the birth of my daughter,' he says. 'It was totally unplanned, but she managed to arrive just as I'd finished the book, and it was rather wonderful. I can't think of anything more likely to dispel that sense than the birth of a child.'

Miller doesn't live with his partner and baby daughter, Frieda Kathleen. He denies this is a trait of a writer determined to be immersed in solitude.

'If I left writing the book, even for a few days, it would build a kind of skin over itself which was quite difficult to get through,' he says. 'When it comes to writing I like to keep it open and that means getting to it at some point every day over that period.

'But there're also occasions when exactly the right thing is not to do it - walk away from it and do something different. Part of the skill of doing it is recognising when not to do it. If there's ever an invitation to go out, I'll go out every time.'