NO one could accuse BBC spy drama Spooks of being predictable.

Ever since its first episode, when a member of the core team was killed off by being dipped headfirst into a vat of boiling chip fat and then shot, the gritty show has developed a reputation for thrilling TV.

Since then, over six series, we have seen MI5 agents come and go; whether they're bumped off at the most surprising moment, as Lisa Faulkner's character Helen was back in the show's debut episode, move abroad to evade jail like Zoe (Keeley Hawes), or be forced to leave the Security Services like Tom Quinn was in the third series, no one in the world of Spooks is ever safe.

When the latter character, played by Matthew Macfadyen, departed in 2004, in came Rupert Penry-Jones as Adam Carter - the new golden boy of MI5's elite counter-terrorist unit, Section D.

The handover was smooth for viewers, with one strong leading man replaced by another.

Adam's arrival heralded a change in tone of the award-winning drama too.

While Spooks had been a slow-burning drama about the ins and outs of life as a spy, the show became more action-packed. Storylines had even more impact and episodes rattled past at a faster pace.

It's no secret Penry-Jones is leaving Spooks behind, but, thanks to a rather MI5-esque policy from the programme-makers regarding storylines, no such details will be given away here.

We can reveal that Richard Armitage is joining the cast, however. He's probably best known for playing Guy Of Gisborne in BBC One's Saturday-night series Robin Hood.

A housewives' favourite, he certainly has the looks needed for the part - the very mention of either Macfadyen or Penry Jones' names can elicit swoons at 50 paces - so the eye candy quota is being kept high.

Standing well over six feet and of athletic stature, Richard also has classic leading man presence, although the question of how he feels about becoming the poster boy of one of the Beeb's most-loved shows is met with raucous, incredulous laughter.

As he explains, he's not too short on enthusiasm for the part either.

Clearly, it wasn't the thought of being plastered on bus sides all over the country that attracted him to the role.

"There's nothing I can really compare Spooks to on British TV," he begins, once he's stopped cackling.

"Once they bent my ear, I was gagging to be involved."

But 37-year-old Richard, whose other credits include The Golden Hour, Cold Feet and North And South, wasn't always so sure about taking on the part.

"I did take time to think about the job, and I did hesitate for a second.

"I was a big fan of the show when it started, but because of being busy, I hadn't seen it for a while so I didn't know if it had gone bad.

"It's also in its seventh series, so I wondered 'Is it on its way out?'. Taking over from Rupert is such a tall order," he adds, "so I was really conscious of that and didn't want to be responsible for bringing the series down. If it closes at the end of series seven, it's not a good thing to have on the CV!"

After catching up with the series on DVD and being shown rough outlines of the series seven script , Richard was happy with the quality of the forthcoming run and said yes.

Next, he had to work out how he was going to continue with the filming of Robin Hood on set in Budapest, Hungary, which overlapped the shooting of Spooks by a number of weeks.

Thankfully, the powers-that-be made it work, and Richard was able to do both jobs.

He'll play Lucas North, a top MI5 agent returning to Britain after eight years detention in a Russian prison.

"He was caught, which is the worst thing that can happen in that world," says Richard, who, at the time of this interview, was mid-way through filming on Spooks.

"I haven't seen the script for episode eight. They're so secretive here," he says.

Lucas is then freed by his Russian captors in part of a trade-off brokered by head of Section D, Harry Pearce [Peter Firth].

"When he gets back, he's pretty wrecked, you know, really quite damaged. Lucas is described by Harry as being malnourished and high on adrenalin, which he is - he looks terrible."

Taken back to MI5's Thames House base, Lucas is awaiting debrief, but is thrown back in at the deep end when, on Remembrance Day, a group of Islamic extremists threaten to execute a British soldier returning from Afghanistan.

A stickler for realism, Richard decided he wanted his character to look emaciated during his early scenes, but lean and sinewy rather than weak.

"There's a fight scene in the first episode, so he had to look like he could fight.

"I wanted to slim right down for it, but the producers held me back from that. In the end, I lost about a stone and stepped up my training to get the right look."

Looking at tall and slender Richard it's hard to imagine where he found a stone to lose, but the fact he did highlights the sort of actor he is.

During our conversation he reveals a complex backstory invented by himself for the character. Many actors do this, but more unusually, Richard talks about the future he has mapped out for his case officer character.

"It's like when you look at a tree," he explains.

"There's the same amount of tree under the surface as there is on show, but in a person's past are clues to their future.

"I had a drama school teacher who always asked what my characters dreamed about. What would they like to be doing when they were 50 or 60? That's how you create a well-rounded person."

So does that mean there will be more Spooks with Richard as the star?

"Yes, I'm definitely keen to do more," he says.

"I just feel like I'm scratching the surface with this, and I still feel there's more mileage in the character than just this one series.

"I want to find out more about him, Lucas is really interesting," he continues.

"Then again, I don't know what's happening at the end of the series. As I said, I haven't seen that eighth script yet."