Interview: Andy McNab, soldier and best-selling author

• Killing? It's just a job for Andy McNab. The highly decorated soldier and best-selling author feels no fear; rather he enjoys the excitement of conflict. But is this physically powerful man actually an emotional coward? And is anyone brave enough to ask him?

What if someone got run over and killed in front of him? "Well, they're dead, aren't they? What's the point in being upset?" Married five times, he talks with equal detachment about ex-wives. What does his fifth wife think of him? "That I'm a dickhead." Thing is, he's just been tested for an experiment at Cambridge university and the parts of his brain governing empathy and fear were both visibly underdeveloped.

How did he feel about that? "Great," he says. We both laugh. Even the psychologist joked about it. "He said, 'You don't care do you?'" And McNab's wife? "She just said, 'I know'. She's known for years. That's why she says I'm a dickhead."

Blue eyes … dark hair … the craggy side of handsome. Friendly and engaged. McNab is never photographed openly – nor does he use his real name – because of his intelligence background in Northern Ireland. But we meet in a London hotel and he talks fast, in a Cockney drawl, with the openness of a man who's figured things out and doesn't much care what others think. Clues to his extraordinary levels of detachment are all there in a complex life story, from his abandonment as a baby to his capture and torture in Iraq. There are clues in his books too. The man who entered the British army with a reading age of 11 has just published Zero Hour, the 13th Nick Stone novel. Stone, a tough, independent intelligence operative, is a largely autobiographical creation, McNab admits. It's easier that way. He describes Stone as "an emotional dwarf". But he also says he has the capacity to be "soft as shit". Draw your own conclusions.

Zero Hour has all the fast-paced elements of a typical McNab thriller, with human trafficking in Eastern Europe and international espionage involving secret and sophisticated technology. But the human side of the plot is Stone being told he's dying of a brain tumour. He's not frightened, but he begins to think he'd better look around at life, turn his attention to things he's ignored: art and beauty and love.

McNab knows the transformative effects of facing death. In 1991, he led Bravo Two Zero, one of the most controversial British SAS missions in post-war history, when eight soldiers were dropped inside Iraq with the instruction to disable scud missiles. The group found themselves immediately out of radio contact and three died as they tried to escape to the Syrian border. McNab, who was awarded the Distinguished Conduct Medal, was captured a few miles from safety. Imprisoned for six weeks, he was tortured by his Iraqi captors.

Was facing death a life-changing event for him, as it was for Stone? "It's interesting because it wasn't a big sea change. I still carried on for three years after I got out, working in intelligence services, so that wasn't the end of it."

The physical pain of torture is obvious. But there must also be the psychological torment of uncertainty and anticipation. Which tested him most? "The not knowing. I didn't know how long I was going to be there.

There were five of us kept back in the first prisoner releases so I thought I was going to be best mates with Terry Waite for the next five years."

Looking back, the most important preparation for his ordeal was during training, when he sat for ten days listening to former hostages talking about their experiences. "Prisoners of war, policemen, FBI guys who had been taken hostage by drugs guys in Colombia and Mexico," he explains. "And I listened to this marine, a Phantom pilot from the Vietnam war. They flew 80 missions and that was the end of their tour, and he had done 77 when he was shot down over Hanoi. He spent six years in solitary confinement. Every major bone in his body was broken. He had no muscle mass in his arse, no hair, no teeth, but he was still alive. And what he said was, 'You can't do anything physically about your predicament. If you get belligerent, they just put another couple of guys in. What you need to do, is keep mental integrity.' And I thought, 'Well, if he can do it I can do it.'"

The pilot had built a house in his head and painted it. McNab found that boring. Instead, he mentally took his daughter for a walk across the park to the sticky bun shop they used to visit for a coffee, before walking home. (Interestingly, in his autobiographical writing, it's only when McNab talks about his daughter that he loses his emotional reserve.) But did he ever doubt his own ability to cope? "No," he says instinctively. "We got lined up in a mock execution just before we were released. I was standing next to a US marine pilot called Joseph. We heard all the weapons cocking so we thought we were going to get dropped. Joseph started praying, mumbling away to himself, because he had a grandson he had never seen so he thought he'd say a prayer. Then a couple of lads started sobbing. But actually, the vast majority, pilots included, thought f*** it, just get on with it because you've got no control over it. You can't do anything about it so f*** it. You're not going to cry."

He might not want to show fear. But did he feel it? "No," he says defiantly. "What can you do? There's nothing. Are you going to turn round and cry? No. F*** 'em. When we got handed over to the Red Cross, the first thing everybody wanted to know was, who were the f***ers who started crying and begging? That's the last thing you want to hear. They really got a hard time."

So when was the last time he did cry? He thinks for a moment. "Last May," he says. His daughter's graduation from university. She's the first in his family to go to university. Not that he told her he cried, of course.

Is there anything he is frightened of? "No," he says, but then seems uneasy in case that sounds like a boast. "It's not like a big macho thing," he says. Then in a moment of self-awareness adds: "Part of my problem in life is that I don't give a f***. I see it so clearly – that's half the problem."

McNab was abandoned at birth, left in a Harrods carrier bag on the steps of a hospital. His adopted mother was a cleaner at a children's home who thought she couldn't have children. She took home one child, who would become McNab's older brother, then McNab, though in her late thirties she had a surprise child of her own. "It wasn't exactly a middle-class upbringing and they wouldn't be allowed to adopt kids now," admits McNab. "But you don't need to be in a nice three-bedroom semi. All you need is people looking after you."

As a child in south London, though, he grew up in a house with little money, skipping school and involved in petty crime. But a family was better than a children's home, he thinks.

He has always insisted that being adopted was no big deal. But is it perhaps because of his background that he has that sense of dislocation, of not caring? "There is that theory, isn't there?" he says. "That theory that there is always this wanting." Never mind the theory – what does he think?

"Do you know, I felt quite fortunate. I was adopted when I was five and my older brother was adopted when he was 11. He was a battered kid. He had burn marks on him because his mother burned him with an iron and I used to look at that and think, you know what? Everything's fine. I'm all right. I wasn't abused or beaten up. I was just left."

Having no idea where you're from might create a sense of loss for some but McNab says, for him, it was a feeling of freedom and independence. "I always knew, even as a kid, that I wasn't going to be at home for long, that I would be gone as soon as I could. Not because it was horrible. It wasn't great but it was all right. But it was like a waiting room … I'm off, I'm off, I'm off. And I knew that when I got in shit, I would always get out of it. It's one of the reasons I can't be arsed with insurance." Despite still working for a private military company in Afghanistan, he has no life insurance.

He did once have his DNA analysed but it was only because his daughter lived in America and you get special tax status there if you have any Native American blood. Amazingly, he did, though the Americans still wouldn't grant him American Nation Status. He now wonders if his father was an American soldier, or one of the Native American population from outside Manchester, who settled there after travelling with Wild Bill Hickok's shows. But he insists he doesn't waste much time on speculation.

His elder brother tried to find his biological parents in his thirties. By then, he had a couple of children of his own but had also decided to adopt one. His adoptive parents went crazy. "He wanted to find his mother to complete it all. He found her and my mum went mad."

McNab feels comfortable with his adopted parents – he has bought their house and looked after them. "I don't know," he admits, "whether it's the fact it would upset my mum and dad," that prevents him investigating his roots. There might come a day? "There might."

He hated school as a child. Even now, when he goes to tell children to stick in with their education, he hates the smell of schools. It takes him back. It was only when he joined the army that he became educated. "The military education service is the biggest adult education service in Europe," he explains. Many who join the army are like him: youngsters who just haven't made the most of themselves. "They're not thick – just uneducated." Education changed him. He went back to read the classics, started looking round at art and culture, and even discovered he liked ballet. But did it change his attitude to violence? "I actually became more pragmatic about it. I like fighting."

It's hard to know how much more pragmatic he could get than saying he "couldn't care less" that, as a 19-year-old soldier, he killed a man in Northern Ireland. As he got older (he's now 50) did that recklessness change? "No, because it's actually quite exciting. That's what you do. You're in body armour running around and you've got a gun … and you're shitting yourself at the time but actually, it turned out all right. There's an incentive to kill because you get two weeks' leave … the first kill of the tour … there's some credibility in that."

Even Nick Stone says you have to wash the blood off your hands quick or it stains you. Isn't he ever haunted? "No."

In fact, he killed a man just four years ago. He left the army in 1993 but still does work for the MoD and private security firms and travels regularly to Afghanistan. "I usually do the good stuff in Kabul. In the middle of this shit hole there's a 35 million hotel, built just for the political and military elite to do their meetings." But sometimes, getting to and from it, you get caught up in bad stuff. "I don't think about it. You have a responsibility to yourself to stay alive and a responsibility to make sure everyone else stays alive. There's no big debate. It's not as if you are trying to reason with these guys, because they've got guns and you've got no control over them. You just get on with it and run away as soon as possible because there might be more of them than there are of you."

He describes himself as agnostic rather than atheist and, for him, the morality is simple. "It is a business. It's not about right and wrong. I like fighting. I think we spend so much time turning our military into victims. Our poor boys – they haven't got this – and actually, if you talk to our poor boys, they're loving it. They're there because they want to fight. If you don't like fighting, get out. If you don't want to fight, we'll chuck you out. It's easy." But perhaps there's a price to pay. Is it emotionally damaging being in the army? "Totally."

You can see why. How can you have McNab's levels of detachment in your day job, then be different in your private life? Maybe that's why he's had five wives. When he left his second wife, he didn't even plan it.

He realised he wanted to go, told her, then jumped out of a window and left. "I was totally selfish," McNab admits, "but I only realised that 15 years later. "I really liked being in the army so it was almost the defining thing. I wanted both. The house, the car, being married, but at the same time I liked going away. I once volunteered to go away for two years and thought that was all right. Of course, it wasn't - but in my head it was."

He is clearly a very brave man, physically. But I wonder if he's an emotional coward. McNab's silence lasts just long enough for me to wonder if he's annoyed. Then he answers with complete equanimity. "Yeah," he says, then thinks some more. "Yeah Yeah. It's getting better but you think, I can't be arsed with all that ..." But his fifth marriage has lasted ten years now. So what's different? "She's smart. She gives me a slap around the head. She's smart and I'm different."

THE Cambridge tests that McNab undertook were part of research looking into the possibility of developing helmets for fighter pilots that would temporarily "knock out" the areas of the brain that process fear and empathy, allowing pilots to do their job more effectively. McNab found the whole area of brain function, and the influences of culture and upbringing, fascinating. "There are many traits that psychopaths have but actually, you could find the same traits in a successful CEO of a multinational company. There's a huge amount of selfishness – they have to have that."

What are the influences that make our brains operate as they do? What's the difference between a psychopath and a CEO? Or a soldier and a murderer, come to that? The helmets being researched sound like a science fiction fantasy but McNab spends his working life looking at things the general public might find incredible. Zero Hour centres on the real-life 'kill switches' inserted into weapons systems that enable one country to switch off another's entire defence system at critical moments. It's what readers like about McNab's book. Having worked in intelligence, he describes the murky word of "deniable operations" with authority. Working for Queen and country? The Queen doesn't know they exist, one of his characters says, and the country doesn't care.

Are the British intelligence services lawless? "Every country has deniable operators," he says. And don't expect politicians to know about it. "When a new Foreign Secretary gets their brief, they don't want to know everything. If you don't know, you don't have to deny it.

They might only be there for a year or less so it's not as if they get the whole deal – and they don't want the whole deal." In America, he says, Obama signed more executive orders on intelligence operations in his first three months than Bush did in his whole eight years.

McNab appeared for Cameron at the last election because he says Cameron "got" Afghanistan and military issues on a level that he didn't feel Labour did. But he claims he's largely apolitical and has voted for both parties at different times. He may have started out life with money problems but he's now wealthy. "It's strange," he says, "but money doesn't really interest me." He has been working on film scripts in the US recently, has international success with his books, and also has his military commitments. But money doesn't drive him. If he focuses on the jobs he's doing, money takes care of itself.

Some people say his refusal to be identified is just a cynical attempt to perpetuate the enigma. He denies it, saying it would be advantageous to show his face because he's had to turn down invitations from shows like Jonathan Ross's. Far more likely is that anonymity suits McNab's sense of detachment. "Yes," he agrees. "Over the years you meet actors and stuff and actually, it's a crap life. They all want it in the beginning and then suddenly they don't and it's too late. I get the best of both worlds."

In this best of worlds, he cares only about those who are immediately important to him. "As far as I'm concerned, all that matters are those who matter – and f*** everyone else. F*** 'em. Who cares?" n

• Zero Hour (Nick Stone 13) by Andy McNab, published by Bantam Press on Thursday, 8.55 on Amazon