MI-5

Rupert Penry-Jones (Adam Carter): "I imagine that real officers must see the series as amusing, but completely ridiculous. The things that we do in an episode take years in reality to come together. Also, we get paid more to pretend to be them than they do - which must be pretty galling! What those guys have to deal with every day is totally extraordinary. Sometimes you'll read a script and think it's too far-fetched; just unbelievable. Then the next day you open the paper and there it is, actually happening. Real people putting their lives on the line in the most bizarre and extreme situations."

Matthew McFayden (Tom Quinn) on leaving after the third season: "At the beginning I didn't think in terms of how long it would go on for. But it was very sad, very odd to go. Inevitably you feel quite proprietorial and think: how can they be carrying on without me? "But I also thought: what else can you do with Tom? In two series he suffered quite a lot and it was really time to go."

Keely Hawes (Zoe Reynolds) on the transition from Tom to Adam: "Tom's behavior has a big impact on Zoe. She thinks the worst of him, she's completely convinced that he's a traitor. But no-one expects Tom to go. It doesn't hit Zoe quite so hard as Danny because things are going so well in her personal life, and her relationship with Danny suffers as a result.

"Zoe's immediate reaction to Adam's arrival is dislike. Anyone who takes over from Tom is going to have a hard time. She thinks he's a bit of a wide boy and he really has to earn her respect... which eventually he does."

Peter Firth (Harry Pearce): "I'm sure life at MI5 isn't exactly how we portray it, we glamorize it, but we provide a window into a world that is effectively secret and closed. We humanize it, give a face to the faceless. The motives of these people are unquestionable. I respect anyone who has a commitment that is so great; they dedicate their lives to it."

Raza Jaffrey (Zafar Younis): "Spooks always goes in at the sharp end. The researchers and writers do a great job to make sure that the series is as accurate, and at the same time as entertaining, as possible."

Olga Sosnovska (Fiona Carter): "In preparation for the role I read the Encyclopaedia of Espionage, front to back. It's the first book of its kind, out for the first time. Full of facts, jargon … I probably retained about 2% of it!";

Jenny Agutter (Tessa Phillips): “It’s great to play someone very different from you. Morally, I'm a bit prudish, but Tessa is very cynical. She’s very solitary too. She has all the trappings of a successful bachelorette – a modern apartment with loads of gadgets, gourmet takeaway meals every night and champagne permanently in the fridge – but she never lets anyone get too close to her. She’s just looking after numero uno.”

Anna Chancellor (Juliet Shaw): “Juliet is a powerful woman who has no cracks. She’s very driven and ambitious; unyielding. I am a counterpart, or the opposition, to Harry and the whole team.”

Nicola Walker (Ruth Evershed): "I like Ruth, but I don't think I'm very much like her. Although I have become quite worryingly obsessive about Ruth's desk and personal props. When we had to start hot-desking I couldn't cope at all at first. I came over very 'Ruth' and I remember saying, “I can't work there!” But I got over it!”

Hermione Norris (Ros Myers): “Ros Myers has the Secret Service in her blood. Literally. Her father is a business mogul with high-ranking political links, and the world of politics and espionage is all she knows. And it's all she wants to know. She's married to her job, utterly uncompromising, ruthless and would do anything for what she believes to be the 'greater good'. She's really good at her job - and she knows it. All of which makes her great fun to play because she's not like me at all!"

David Oyelowo (Danny Hunter) on Tom's disappearance: "Tom was very much Danny’s mentor, so it shakes his faith in the service. The reverberations of that carry on throughout the whole series. When Tom leaves there's a massive void. Danny and Zoe become much closer because they've gone through something together, and Zoe feels for him but isn't in love with him. Their emotional lives bleed into their working lives, but for her there's someone else."

Life in MI5
by ex-MI5 officer and Spooks research consultant Nick Day

MI5 officers operate at the coal-face of the information age. Despite recent moves towards openness - including the creation of a website - MI5 remains an impenetrable world and the Spooks team faced a real challenge to portray it realistically. Step up Nick Day, an ex-Spook who now runs a corporate intelligence agency called Diligence. Thanks to Nick's advice, says Exec Producer Stephen Garrett, Spooks is soaked in atmosphere: "We're not giving away the big political stuff of MI5," he says. "Instead we're like one of those mischievous programs that tell you how magic tricks are done."

Nick Day joined MI5 from the Special Boat Service (SBS) in 1996 and spent two years in the counter Middle East terrorism department. This is his account of life in 'five'.

"I joined for the challenge and the excitement. 'Intelligence' is defined as 'Assessed Information'. Whether information is gathered covertly or from public sources, everything an MI5 officer does is geared to assessment - judging the relevance, the truth and risks contained in intelligence.

"Naturally, Spooks employs artistic license, and the characters' operations are extremely varied. In real life, an officer works for one specific department for two or three years like a counter-terrorism or counter-intelligence department. But there are also departments which provide resources across all target areas like the 'Watchers' - surveillance experts - and the 'Buggers and Burglars' who break into buildings and place bugs.

"A General Intelligence Officer could work on up to four or five operations at any one time. They will all be at different stages: for example you could be dealing with the drafting of a warrant to tap a phone line in the morning and then heading out to work on recruiting an agent undercover.

"To work undercover, you need an alias. Long-standing officers could be running up to 30 aliases. Building an alias is a very long process. As well as documentation, you need a history or legend to make it credible (where you went to school, what jobs you had, where you have lived) and 'backstops' - people who can vouch for your story if a target gets suspicious.

"There are in-house resources to help you create a legend including a costume department and a props department, which produces the essential minutiae known as 'wallet litter' - ticket stubs, travelcards etc. It's important to 'sanitise' your kit and clothes, meaning you keep each alias separate. You lock one identity away before taking up another.

"Initial training takes two months, but officers continue to attend courses throughout their careers in anything from lock-picking to the use of codes. Instructors like to test their pupils. One classic training trick is to put a cat inside the door of a building so that it escapes when you break in and you have to spend hours retrieving it. Wet concrete behind a door so that you leave footprints is another favourite.

"There is a lot of paper work as well. MI5 is a supremely careful and thorough organisation. Every operation, no matter how small, must be approved through the right channels and there are a number of legal controls. For example, you have to be very careful when recruiting a potential informant, or agent. You can't encourage someone to break the law.

"MI5 agents have a wide variety of motivations. Some are in it for money, but the majority have a sense of moral responsibility. The aim is to recruit long term agents. For deep penetration of an organization, MI5 might recruit ';clean skins' - people who haven't yet joined the target group. Other agents are already within an organization, and others still are simply people with regular contact with targets, like pub landlords.

"Recruiting an agent, known as 'turning' is a long process. You need to research their background and find a way to gain their trust. Approaching a potential agent under an alias with common interests is often more successful than throwing money at them. In one famous case though, an Army Intelligence officer boarded a bus and dropped an envelope into the lap of an IRA guy. He said 'there's 10,000 pounds. I'm with British intelligence. Give me a call'. The Irish man just threw the money back at him. A republican newspaper ran the story and the security services were then inundated with calls from other republicans saying the other guy might not have wanted the money but they'd be more than happy to help!

"Some of the work is incredibly exciting, but civil service wages are not so great. A lot of people leave in their thirties to do something more lucrative. A friend from the CIA and I are now running a corporate intelligence agency called Diligence. Companies employ us for all kinds of information gathering: to establish the background and assets of clients and suppliers, to check out what rival companies are up to, or to assess various corporate risks.

"After September 11th, the corporate intelligence industry is booming. Bin Laden laundered money through institutions, and people are now much more wary about who their clients and suppliers are. They want transparency in their deals which is not always forthcoming. We also advise businessmen traveling to countries like Chechnya and Columbia who may run the risk of being kidnapped. My top tip for paranoid potential kidnappees is to develop awareness. Kidnappers and terrorists almost always watch their targets before they strike. Watch out for people following you and avoid washing your car. That way you can see if it's been tampered with!"