Cold Feet actress Hermione Norris says she felt like a new girl in school when she joined the cast of the long-running TV series, Spooks, but, as she tells Steve Pratt, she had a really good time.

The growing threat of terrorism, fuel shortages and loss of confidence in the Government throws Britain into chaos as BBC1's Spooks returns with a two-parter that sees the country on the verge of anarchy. Enter headstrong, high-flying MI6 agent Ros Myers - played by Cold Feet and Wire In The Blood actress Hermione Norris - who soon makes her presence felt among the regular team headed by MI5 supremo Harry Pearce and team leader Adam Carter.

Norris says that Ros "has the Secret Service in her blood". Literally, it turns out as her father is a business mogul with high-ranking political links. The world of politics and espionage is all she knows, or wants to know.

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"She's married to her job, utterly uncompromising, ruthless and would do anything for what she believes to be the greater good," she explains. "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."

She felt like the new girl at school joining the established cast of the long-running series and admits it was "quite daunting". The reality was that she had a good time. "Everyone has been lovely. And they're so funny. Everyone is laughing all the time. It's almost a problem to get serious when the action starts because you're laughing so much," she says.

There appears to be some sort of spark between Ros and Adam (played by Rupert Penry-Jones). He recruits her from MI6, although she feels she's on an equal footing with him. Determined to establish herself on the Grid, she's quite cool towards him at first. "They have quite a combative relationship," she says.

"They respect each other but both like to challenge one another all the time. They are quite similar in personality and in that, for both of them, the job comes first."

Ros' private life is a bit of a closed book. The only glimpse into her past comes when she's looking after a witness who nearly gets killed on her watch. To save his life without leaving any sort of trail, she makes an emergency call to an ex-boyfriend, who's a doctor.

"That opens the lid a fraction on her otherwise well-contained private life," says Norris.

She has stunt scenes which she loved doing, although she didn't come off too well from one of them. "Nothing serious, just a carpet burn, but it hurt like hell," she says. "But that's part of the fun of acting, the unpredictability."

As new girl Ros arrives, MI5 supremo Harry Pearce - played, since the start of the series, by Peter Firth - carries on regardless. Other characters have come and gone but he's still very much in charge.

"The beauty of Spooks is that you never know what, or who, is round the corner. As soon as the scripts arrive, I check to see if I've made it to the end," says Firth.

"Nothing seems to change Harry though. He's a rock. But bringing in new characters means that the dynamics shift around him. The series just gets better and better. Not because of the cast changes, but because it has found its style. It has a confidence now which it has earned."

He has a bit of seniority by virtue of the fact that he's been with the series for the longest - and he's the oldest - but on set he says he's just one of the actors.

The set is where he pretty much remains. The Grid is his domain, so getting out on location is a rarity. "I like being out and about. It can get rather claustrophobic being stuck in my office all the time. But Harry really is a desk man. He's the brains, rather than the brawn," says Firth.

"The writers are very limited in the action scenes they can script for me. They tend to go to Rupert because he's younger, fitter and better dressed. Let's face it: I'm not going to get a look in, am I? Do I sound bitter?"

The opening two-parter is a very adventurous piece of television, he feels. It revolves around a coup in Westminster which is "a really unusual, but actually totally plausible plot and a very full-on story," he says.

"But the best episode for Harry is episode five. I don't want to give too much away, but it's the end of the road for one of the regulars. And it's utterly devastating for Harry..

"Of course, Harry being Harry, we don't learn a whole lot more about his personal life, but we do discover a bit more about him as a person."

This series sees the relationship between Harry and MI5 desk officer Ruth Evershed develop. It's just seemed to emerge and become part of the storyline, he says. "It was never originally scripted," says Firth.

"We seem to have created that romance. Obviously it has started to be scripted now and, yes, it does develop in this series, but you'll have to wait to find out if Harry, or Ruth, get their hearts broken."

* Spooks returns to BBC1 in a two-part adventure on Sunday and Monday at 9pm