How do you see the role of the sidekick in crime dramas?
"Lewis was invented, TV-wise anyway, as a sounding board for Morse, a way of getting his thoughts out in the open. Where Morse was a hugely cerebral character, Lewis was your everyman, so the audience could look at Morse through him, and my reactions mirrored how people would feel.
"So then John's gone and they asked for two or three years if I would do it [a spin-off] and I couldn't see the point because, as I say, Lewis wasn't meant to be an interesting character in his own right. Over the years we'd built up a back story and obviously people liked him, but whether he would work in a vacuum I didn't know. They had these ideas of what to do with him and the first one was to kill off the wife, which is a bit of a cliché . . . then we brought in the sidekick.
"The thing I enjoy about acting is working with other actors. The thing that bemuses me is people who do one-man shows because I can't imagine what fun they get out of it, just standing there and spouting to an audience. Acting to me is all about interplay, and because of the nature of Lewis, for the most part you're interviewing people, so it's all question and answer. The only time you don't have to ask questions is when you have a bit of interplay between the two regulars.
"They came up with this idea that they wanted a young version of Morse, a very English bloke, Oxbridge-educated, a slightly spiritual character for me to bounce off - and it just happened that I'd caught the last ten minutes of Colditz [the 2005 version with Damian Lewis and Sophia Myles]. And there was Laurence Fox at the end. I hadn't seen any of the story, but I thought, 'He's got an interesting face' - I didn't know who he was. So they brought him in with four or five others and Laurence got the part [of DS James Hathaway]."
"They've evened it up more this year, but it's certainly tough because at the end of the day you're getting a lot of story in and story is: (a) the hardest thing to act; and (b) the easiest thing to look stupid while acting. So I haven't minded not having much to say.
"Kev said that all you had to do was watch out for the moment when they were going to cut to you, which inevitably they do every scene, and then do something in that moment. So that was good advice. Actually, I don't mind the term 'sidekick' at all. I find it quite nice.
"We two probably make up 20 per cent of the show as far as story goes. But it's 80 per cent of the reason why you bother watching it each week - or at least that's my theory."