Robson Green believes the new episodes of Wire in the Blood are the best - and scariest - yet. "I feel that this series is our strongest so far, and we are always pushing the budget so each episode has a filmic quality. I am very proud of it, and I hope our viewers will be terrified and intrigued by it. One of the episodes is full of religious imagery - hanging, beheading and drowning with a millstone around the neck. The killer is someone who's besotted with the artist Caravaggio. Like the slaying of John the Baptist whose head was served on a plate, which is how one of the victims ends up. It's full of symbolism and it's like a feature film in the way it's shot and lit. However, having a conversation with a head on a plate covered in blood, in between takes, was slightly surreal to say the least. He adds: "The feedback we've had from around the world is that people enjoy being scared by the films and want us to push the boundaries a little bit further, without being gratuitous of course. Our aim, through the writing, performances, design and effects, is to create a chilling atmosphere."

Robson thinks it is good that Wire in the Blood doesn't shirk from the truth about serial killers. "Destructive behaviour is not something that drives itself from an outside force known as 'evil', but from a choice made by a human being. The consequences are visually unpleasant. Our programme is not real but what we have to say is important. As long as it's not a gore fest, we don't shy away from the reality of what we're talking about. I don't go around celebrating the violence but I enjoy experiencing the reasons why people behave as they do. It's an interesting learning curve for me - the most educational piece I've ever been part of."

Robson's character, eccentric clinical psychologist Dr Tony Hill, has to learn to work with a new police chief in the series, thanks to the arrival of DI Alex Fielding, played by Simone Lahbib. "She really doesn't want him on board but in the end thanks him for being there. Tony and Alex have to learn to trust each other and work together to solve the murders and their new relationship brings a new energy and warmth to the series."

Adds Robson: "Tony's far more outside the university now and more involved in the police work, as a volunteer. But he's on the periphery. When the crime is solved he disappears. He's never applauded for what he does and he doesn't stand out in a crowd. Just like a surgeon who performs something profound in hospital or someone who's carried out an amazing engineering feat. You wouldn't look at them twice."

Returning to the character of Tony Hill means learning long speeches - a challenge for any actor. "Not many actors have the bottle or courage to stand in front of the microphone for eight minutes and deliver dialogue. It's very complicated stuff - bizarre but interesting to say. Luckily good writing is easy to learn, especially once you understand the imagery, so I talk to the writers and Val McDermid a lot."

Despite his close involvement with the series, Robson insists he never takes the dark subject matter home. "This is fun and it's a job I really enjoy but I know it's unreal. I'm not a psychologist, I don't take it home and if you do, don't do the job. It's the real stuff that lingers for me, like happy slapping kids."

The success of Wire in the Blood, with sales to well over 30 countries, is particularly rewarding for Robson as it is made by his company Coastal Productions. "I can understand why it is a very successful programme because its themes are universal and what Tony has to say is very interesting. It's unusual though, because he's a clumsy character, not your clichéd lead. He travels to work on a bicycle and has his life in a blue polythene bag. It's taken us 10 years to get prime time drama that's shown around the world. Once the infrastructure's created then people are attracted to it. The aspiration is to make movies from here."

Despite his profile internationally, Robson is in no rush to try his luck in America. "I'm offered stuff but it's a whole new ball game. You have to up-sticks and go out there and I have a son in education here. But obviously if they offer me something with De Niro for three months that'd be different!"

Robson lives in Surrey with his wife Vanya, six-year-old son Taylor and stepdaughter Lara. His many credits include Soldier Soldier, Casualty, Unconditional Love, Trust, Touching Evil, Close and True, Reckless, Grafters and Rocket Man. He recently completed new ITV drama Little Devil and is about to start filming a new series of Northern Lights. His hobbies include tending to his organic garden. "I've got a greenhouse with melons, aubergines, marrows, peppers and chilli. I see it as nurturing life which I guess is good for the soul."