So many major characters were killed off during the nine-and-a-half years that Spooks was on television, that Harry Pearce, head of the counter terrorism department at MI5, was pretty fortunate to still be standing by the end.

In fact, he was practically the only member of his team left. During its 10 series, many of the BBC drama's stars were bumped off, including the characters played by Rupert Penry-Jones, Hermione Norris, Miranda Raison, Richard Armitage and David Oyelowo.

Peter Firth, who reprises his role as Harry for the big screen version - Spooks: The Greater Good - thinks that dangerous theme, where nobody was safe, played a key part in the success of the series, which garnered a string of Bafta nominations and wins during its run.

"It had never been done in television. Nobody had ever had the idea of establishing heroes, and then killing them off on a regular basis," explains the Bradford-born actor.

"And when we did it the first time in the second episode with Lisa Faulkner, and plunged her head into a deep-fat fryer, the reaction was so extraordinary, it became clear this it was very powerful dramatic tool.

"What it meant for the viewers was that all bets were off. If you had an investment in a character and they were in trouble - they were really in trouble, so you'd better watch, and that led to people being fascinated by the show."

The drama came to an explosive end in 2011, however. Harry, one of the last men standing, was still in charge of the team, dubbed 'The Grid' - so when it was confirmed that a spin-off film would be coming out, fans would surely have been up-in-arms had Firth not been on board.

Waiting in line to discuss the film with the 61-year-old, is almost as nerve-racking as being interrogated by MI5.

But, in real life, he is nothing like the super-serious authority figure he plays.

He's actually an extremely jolly fellow, who leaps up from his seat to greet me warmly as I enter the room, and is as funny as he is friendly.

Playing Harry again after three years was, quite literally, like putting on an old pair of shoes, he insists.

"I wore the same shoes for 15 years. I bought them in [London's] Jermyn Street, a good pair of brogues, and as soon as I got them on for the film, there it was - job done, back in the groove."

Spooks: The Greater Good also stars Game Of Thrones heart-throb Kit Harington, as The Grid's newest recruit Will Holloway, who is called upon to investigate Harry's mysterious disappearance in the midst of an impending terrorist attack on London.

Directed by Bharat Nalluri, who also worked on the TV series, a film was always on the cards, right from day one, says Firth.

"It always wanted to be a film. The scale of the series was much bigger than normal TV drama, it always had aspiration to make it to the big screen," explains the father-of-four.

"So we took the DNA of the original TV show and just put it in a much bigger frame, with better, more exotic locations, more fabulous stars. So it was a natural progression. I wasn't surprised, I was just delighted."

The film has kept the gritty realism that helped make Spooks such a hit. Fans can expect explosions and heart-in-your-mouth chases, but no unfathomable CGI effects.

Firth describes it as "believable, but still cinematic and sexy", a nd filming on location in London meant there was plenty of exciting action for the cast and crew too.

"We don't have any control over the streets, so we did a lot of gorilla filming," explains Firth. "We'd hide cameras on rooftops and in windows of buildings, and I'd be hooked up with sound, which looked like part of the spy equipment anyway, and get instructions.

"And we'd film scenes in busy streets, with people completely unaware we were doing that. It's a great way to work, quite exciting."

It went smoothly, most of the time - apart from when a fan spotted Firth in the middle of a scene and interrupted to ask when the film would be coming out.

Playing a spy in a big-screen thriller at the age of 61 means the actor ranks in the league of older action stars currently storming the box office, alongside Liam Neeson, Sean Penn and Sylvester Stallone.

Firth is not sure if he likes being put in the same category as Stallone, though.

"I haven't had quite as much work as he has," he exclaims with a laugh. "Perhaps I should!"

But he admits he's happy to embrace the tag.

"Hey, we're all living longer. There's no age-limit on being fabulous!"

:: Spooks: The Greater Good is released in cinemas on Friday, May 8