Can you tell us what happens in the first couple of episodes of Strike Back?
"Episode one and two are about a hostage-take. A Sky News journalist, Katie Dartmouth, is taken and held hostage in Iraq and John is sent in as a last resort to rescue her. But it becomes sort of a love affair. Even though there's no physical attraction! When I say love affair, it's a sort of transcendental thing. These two become so connected to each other because of this extreme experience that they go through. When he finds her she's very damaged, and he has to deal with that at the same time as being quite brutal to the enemy in order to rescue her."
What attracted you to the role?
"I think we've seen a lot of war movies and television programmes where we see killing as quite an easy thing and something which is cinematic and dramatic. I wanted to look at what happens when a soldier is disconnected from killing because of his training but then when he tries to reconnect himself into his family life and his children, how the two marry together and if that person can function as a soldier and a man."
And John Porter has a lot of guilt, doesn't he?
"It's not that he feels pain every time he kills the enemy, it's that he recognises that in order to do that he has to disconnect from himself. I think that becomes a big burden on him which is almost crippling. He has to mop himself up after every traumatic event. It's not the same as post-traumatic stress disorder. I haven't necessarily seen that explored before."
So how do John Porter and Lucas North from Spooks compare?
"I think interestingly they marry quite well together. Lucas is MI5, he's very London-based, it's homeland security and he's a problem-solver. He's a cerebral man, he's a thinker, and he deals with the immediacy of what's happening in London politically. John Porter is MI6 and he's right at the cutting edge, right on the front line, in deep cover, in far more dangerous [situations], and he's sort of the weapon. He's MI6's weapon. So they are two extremes, really."
And you're filming the next series of Spooks at the moment. It doesn't have a problem with killing off its main characters, does it? Does that worry you?
"No - I think it's why Spooks is coming into its ninth series and probably will be going into a tenth and an eleventh. I think it deals with its exits really interestingly and when it brings new characters in, it furnishes the show with something really exciting. It's good to know that the show will go on despite a single character and I'm sure when it's Lucas's turn, he'll go out in style."
So you're not flicking through all the scripts trying to see if you're still alive?
"You kind of do. There is a code of honour where they'll pull you in and say, 'OK, we've decided to do this with your character', but you're still unsure. There's a few surprises in there."
What's it like working with the new cast members like Laila Rouass?
"She's playing a character called Maya who is an old flame of Lucas's, possibly his first love, so that's been really interesting. She plays a doctor. We were shooting in a hospital on the South Bank the other day and it was like those Carlsberg adverts - if Carlsberg did hospitals, what the hospital would be like. It was super clean, and Laila Rouass walked down the corridor as a doctor and I was like, 'This is definitely a Carlsberg hospital'."
Lucas had a difficult relationship with Sarah Caulfield last series. Will that affect his ability to trust Maya?
"Yeah, I think he's had a string of quite disastrous love affairs. This one is particularly different and it's quite hard to tell you without revealing too much of the story but Lucas isn't quite who you think he is and she's part of that story, so there's a whole other character that's contained within Maya."
How was that for you, having this whole other side to Lucas?
"It's interesting because you have to just rethink the character and retrace your steps and go back and make sure that everything fits and rework things that don't work. It's very exciting. And I think this series is very much about identity. All of the characters from the very beginning are not what they seem. All of the new characters that come in actually turn out to be slightly different from what you think."
And will Nightingale be a part of that or is that over now?
"Nightingale comes through as a thread with the old home secretary that was in the last series. That thread is still alive, but it does come to an end at that point and then we have a new problem. Albany. I actually don't know what it is yet. I'm chasing this file called 'Albany' and I still don't know what's in it yet. I think it's something to do with genetics, but they won't tell me. It's sort of top secret."
Lucas has disobeyed orders and done his own thing in the past. Are we going to see more of that?
"He does have to go off the radar quite a lot this series, but I think he's made a habit of doing that so it doesn't raise too many eyebrows. It's just the way he operates. But it's quite a good way of concealing your tracks. If you're used to being a maverick, then people don't get surprised when you start acting strangely. And he does behave quite strangely this year."
Moving on, you were also in Robin Hood. A lot of fans were really disappointed when it ended. Do you think it was the right decision to cancel it?
"I was disappointed. I felt that it probably had life beyond three series. At the back of my mind, I still think that they could resurrect it at some point. And it's a shame that it's not in production at the moment as the Russell Crowe movie comes out, because I'm sure there'll be a bit of Robin Hood fever. Although I probably wouldn't want to be competing against that big-budget movie! I still think that there's a passion for it and a love for it, so it was a shame that it finished. I loved doing it, it was brilliant. I'd definitely do a spinoff, a Guy of Gisborne spinoff!"
Strike Back begins tonight at 9pm on Sky1.