By Tom Geoghegan and Dominic Casciani
Denis Donaldson spent two decades working as a British agent while within the higher ranks of Sinn Fein. Living a double life can be a perilous existence, and it's not just secret informers who are doing it.
Double lives are a recurring theme in Hollywood films or blockbuster novels: one slip - a throwaway remark which blows the cover - could be fatal.
In the UK there have been many high-profile cases of people risking their lives to spy on paramilitary organisations in Northern Ireland for the British government.
Denis Donaldson is the latest figure to be unmasked after leading a double life of informing for the British while being at the heart of Sinn Fein.
Far from normal
Sean O'Callaghan was another such informer. An IRA member, it is claimed he helped ruin some attacks, including an alleged plot to target the Prince and Princess of Wales.
Since his release from prison for the murder of a policeman he has lived at a secret address in Britain.
Denis Donaldson said the charges were "politically motivated"
But he has declined to join the government's witness protection scheme and regularly talks about how his double life was far from normal.
"You live inside your own head, always aware that one wrong word or action may lead to the torture chamber," he once wrote.
"There are as many reasons why individuals become informers/agents as there are people in the world.
"Who really knows? Money, a perceived slight, fear, a desire to avoid a prison sentence or to save a loved one from such, the misplaced sense of power that comes from playing both sides of the fence or a combination of these factors.
"In some cases - as in my own - there is even a need to make amends for wrongs committed in the service of a terrorist organisation."
Double lives come in various guises - and not all as dramatic as that of Denis Donaldson.
Agencies such as MI5 and the CIA are known to regularly approach journalists. If the request for assistance is accepted, it will mean that the journalist starts to live a minor form of double life: informing the public - and the spooks, but keeping the relationships hidden.
But Dr Colin Gill of Psychological Solutions says the web of deception required to run something as mundane as an extra-marital affair, can be a Herculean task in the mind of the subject.
"There's very little empirical research because, by definition, those who are good at leading two lives never get found out," says Dr Gill.
"The change in mobility with modern transport made it easier for those who are willing to lead two lives - the private car has been available from the 1960s, for instance. I knew of one man, a very successful businessman, who effectively had two wives, living half-a-mile apart.
"He would spend a week with one, and then a week with another. Because he was successful, he had bought them their own houses, paid the school fees and so on. The lives were entirely separate. They both knew what was going on but clearly loved him enough to tolerate it."
Trace of technology
But technology means this "Golden Age" of duplicity and deceit may end, says Dr Gill.
Many of us expect mobile phones to be on all the time. But if you choose to have two phones, one for Lover A and one for Lover B, what number does work have? And when work calls the "wrong" phone, what happens when Lover B picks it up first?
Con-man Frank Abagnale, portrayed by Leonardo DiCaprio, led a double life
Other technologies are also playing a part: a cashless society that automatically takes payment for a congestion charge in London or future use of a motorway under road pricing may be pretty hard to conceal, he predicts.
"If you are leading two lives, you are essentially creating two separate soap operas with a separate cast of characters.
"The problem comes when the number of people involved in each life increases - the task of keeping the lives separate grows exponentially because each person you know has a set or relationships which may overlap. You have to keep these apart."
Glenmore Trenear-Harvey, an intelligence analyst who is also a personal friend of Sean O'Callaghan, says the former IRA man is constantly looking over his shoulder.
"Every time we meet [Sean] sits in a corner somewhere with a rear entrance as well as a main entrance, and he's constantly looking at the door to see who's coming in."
Accident of fate
But you don't have to be a secret agent making conscious decisions to act one way or another to face the pressures of a double life - as Frank Abagnale found out.
He was the true-life con artist played by Leonardo di Caprio in the film Catch Me If You Can. In the film, Frank is subconsciously willing the authorities on to bring him to book, tired of the life he is leading.
"Those we know about drift into it by accident," says Dr Gill. "It's not a good life choice. But those who are successful at it become serial creators of new lives.
"There was this surgeon in the Korean War who invented a thoracic procedure. He wasn't really a surgeon and only came up with it because he didn't know what he was doing. When he was found out he disappeared.
"He's probably in his 70s now and may be just living down the road. We still don't know if he was American, Canadian or British - maybe he was all three."