How does it feel to be joining the cast of Spooks?
"It's been much more fun than I thought it would be, considering it's such a serious show. There's a lot of naughtiness that's been going on! The usual banter. I knew Hermione [Norris, Ros] from Cold Feet so we hit it off straight away. Peter [Firth, Harry Pearce] has been there from the beginning and he's a bit like the father of the show. He took me under his wing and looked after me."
What can you tell us about your character, Lucas?
"He used to work for MI5. He was a predecessor of Tom Quinn and was Harry's protégé, the best in his field. He went to Russia and got caught and banged up for eight years. He's tortured, interrogated and broken and then he mysteriously finds his way back. It's cultivated by MI5 to get him back and no one knows why. He comes back with dirt on his feet from Russia and spreads it around a bit. They're suspicious that he's a double agent so that [plotline] goes through the series."
Were you a Spooks fan before doing the series?
"I saw the first two episodes of the first series. I saw the chip fryer incident. I really like how they're not scared to kill off big protagonists. Since then I haven't watched it until they started talking to me about being in it. They sent series three, four and five and I sat down thinking 'I'll watch a couple', then it's 4 o'clock in the morning and I think, 'just another one'. I did get quite hooked. I like the way that it's current and politically mobile. They're writing episodes which are finance-based. There's stuff in there that's pretty clever because by the time it airs the state of the country is going to echo what's going on in the show."
Any nerves about stepping into Matthew Macfadyen and Rupert Penry-Jones's shoes?
"I was pretty freaked by that idea because they're both actors I really admire. Rupert's done such a good job and has a big fanbase and I was worried about it. I still am a bit. The way that it's worked is the character hasn't directly replaced him. It's one character leaving and another arriving and a big re-shuffle goes on. All I can do is go in and do my best work and hope that the fans of the show like it."
Has it been fun doing stunt work?
"Rupert set the bar by doing his own stunts. I saw him roll over the bonnet of a car in series six so I froze the TV to check and it was him. I asked him how they insured him to do that! I couldn't possibly not do my own stunts now. I've done fight sequences and a torture sequence, which was a bit scary. It's different to Robin Hood. That's fantasy fighting, but this is proper hardcore. You beat the s*** out of someone and it looks real."
Do you prefer playing a character like this or someone more sinister like Guy of Gisborne?
"It's great fun playing the baddie, I have to say. The fun side of playing the baddie is that while you're doing it, it's great. The reaction you get back is frustrating because everyone assumes you're like your character and despises you for it. Playing the hero, you get a great response. If someone tells me a character is good, I'll look for the bad stuff. If they're bad, I'll look for the good stuff. I don't see them as either black or white."
What did you think when you found out you'd be killing Maid Marian?
"When I read that I phoned the writer and said, 'What the f*** are you doing?' He said they wanted the most shocking storyline that's going to rock Robin Hood. This is what they decided to do. I still think they're playing with fire, but in a way they've shown an audience they're not scared of shaking up the show and reinventing the show. It means they can go anywhere with it now."
What's going to happen to Gisborne now?
"I think he deserves his comeuppance, to be honest. What form that will take, I don't know."
Spooks returns Monday, October 27 at 9pm on BBC One.