How does your character John fit into the plot of Unforgiven?
"It's almost accidental in that he lives in the house where this murder took place, although he doesn't realise this at the time, and [Ruth] turns up standing on the wall outside. He goes out and says 'can I help you?' and she says 'I used to live here'. She's not your normal free-talking, charming young woman, but I think this is what draws him to her. He invites her inside and she tells him she has a sister whom she has lost contact with. His wife founds out what she did and throws her out, and later John has a kind of cathartic moment where he realises that he can't live with that, so he goes after her to help her find her sister. His point of view is that you can't throw away someone's life on the basis of what they do in five minutes or one night."
How does Ruth behave around him?
"Well, she's been in prison for sixteen years so she's become a very institutionalised, dysfunctional person who is struggling to find her way in the real world."
What appealed to you about the role?
"A couple of things. One is that it's a Sally Wainwright script and I think that she's a great writer so I wanted to do it from that point of view. And I think [John] is also someone I would like to aspire to be like, because he basically does the right thing."
Was it a conscious move to do something hard-hitting and serious after your stint in Spamalot?
"Not really. This is the sort of thing I should do after seven months in Spamalot, but it was not a deliberate move. It's a nice idea to think that one is going to decide to do a certain sort of thing and I'm sure there are actors who are able to pick and choose. I find it very fortunate getting a part like this through the door. I have actually turned down more stuff in the past year than I have for a long time. But it was nothing that was good, it was basically just rubbish scripts!"
Will there be any more of The Last Detective?
"No one has said it's finished, no one has said 'we've cancelled this', but they haven't made any more so I'm assuming we're not. I think it's a shame because I think it was a good series and it was getting into its stride. It was more popular than people realised - I'm asked constantly about it. We hit a bad time [at ITV], a crossover time when people were coming in and obviously wanted to make an impression with their job. I think The Last Detective was an unfortunate victim of that. But it's never too late."
There's a definite trend in television for reviving old scripts and giving them a modern makeover. Do you think there's scope for All Creatures Great And Small or A Very Peculiar Practice to come back?
"There was a chance - somebody dug up an old All Creatures Great And Small script but [the BBC] didn't seem keen on doing it. Maybe they just thought we were too decrepit, I don't know! But they found an old Christmas episode which they'd never done, which had been commissioned by Johnny Byrne, who has since died, sadly. But the BBC didn't seem to be keen on it at that particular moment, although I thought it would be rather a good story. It was about a year and a half ago."
You returned as The Doctor for the Children In Need special 'Time Crash', written by Steven Moffat. Now that he's taking over as the Doctor Who boss, are you hoping for another outing?
"No, not really. If he ever came up with an idea which would involve me returning I would certainly do it, but I can't see it happening and I'm certainly not waiting on tenterhooks for it. I know him quite well now... but it's unlikely I think."
You've said that your young children are big fans of the new Doctor Who. How did your return go down?
"Well, they don't know any other world in which their dad is not in Doctor Who, so they're not as impressed as their friends are. We had David Tennant around the other day and they were almost unimpressed with him, I have to say! That was really extraordinary - it was almost like he didn't exist, it was very weird. My son Louis had a birthday party and Georgia [Moffett, Davison's daughter] was coming to his party and she turned up with David Tennant and every other child in the garden was like [makes shocked face], but my children were like 'I've met him before'."
Unforgiven begins Monday, January 12 at 9pm on ITV1.