(CNN)Whenever Matthew Dunn goes to a restaurant he requests a corner table with his back to the wall.
This is how you become a professional spy
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This habit is a legacy of nearly six years spent as an MI6 agent in the mid-1990s, taking part in undercover missions across the world.
Dunn was a field operative in countries where, if captured, his life would have been in danger. After completing 70 missions he decided to draw a curtain on his espionage days, and has since forged a successful career as the author of the Spycatcher novels.
So how do you take a first step toward what is surely one of the most unusual jobs in the world?
"In my day I was approached at university by a talent spotter," he says. "Back then MI6 had a whole network of talent spotters spread across the whole country, predominately but not exclusively in universities. Essentially, that was pretty much the only way to get in, i.e. they approached you, not the other way round," he adds.
Intellect, interpersonal skills, powers of persuasions and judgment are all tested, as well as the ability to operate both alone and in a team.
And if that sounds a little run-of-the-mill, you also have to be able to drink a KGB agent under the table.
"One exercise was meeting a former, high-ranking KGB officer. The brief was really straightforward -- he's requested a meeting and you've got to go and find out why he wants to see you. Word of warning: He has already polished off a lot of vodka and you have to match him on that, otherwise there will be a credibility issue. Also, you can't take notes during that meeting because he's really twitchy about that, so you have to memorize, with pinpoint accuracy, anything he says, while at the same time match him on a drinking level."
Unsurprisingly, there was more than one occasion when young recruits were violently sick, says Dunn.
In their final test, recruits are taken overseas and immersed in a doomsday scenario. It's so realistic, Dunn says, that many think it's not actually an exercise.
"Independence of thought is crucially tested," says Dunn.
In fact, self-confidence and decision making are essential for survival in challenging situations, because most times, there will be no one who can come to your aid.
"I always think of MI6 operatives as generals without armies. They're the general of themselves," says Dunn.
One of the questions candidates were asked while Dunn worked for MI6 was who they looked up to.
"The answer they're looking for is no one," says Dunn.
"Message within is that if you look up to someone, then naturally you perceive yourself to be beneath that person. They are not looking for characters like that, they need people who are not arrogant but can hold their head up in any situation," he adds.
A powerful memory is necessary to stay alive. At one point in his career, Dunn was operating with 14 different aliases, each with their own passport, credit card and background story (including the school they attended and names of their teachers).
"The moment I stepped out of my home in central London, I lived that new identity," says Dunn.
Unlike in movies where good and bad guys are clearly divided, those lines are less clearly defined in real life.
"Spies operate in the gray zone," says Dunn.
"I was working with people, foreign nationals, who were charming and very bright, but I knew their background and some of them were absolute monsters in terms of what they had done."
After quitting MI6, it took Dunn 10 years to adjust to normal life.
"When I'm driving I still have the habit of mentally noting down the last three digits of the number plate if a car overtakes me," he says.
And did his undercover career make him a fortune, a la James Bond?
"It's public service, so no," he says laughing. "Don't join MI6 if you want to make the big bucks."