What is the premise of Crooked House?
"Lee Ingleby plays a schoolteacher who finds an ancient doorknocker in his garden. He takes it to a museum curator - me - to identify it, and I say it's come from a Tudor manor house which is now demolished and which had an interesting reputation. I just tell him about various stories and strange things that happened there."
When you were writing it, did you always plan to take on the role of the museum curator?
"I didn't actually, because sometimes it can look dangerously like an Orson Welles approach because I co-produced and wrote it. I honestly did not intend to, but they were very keen for me to be in it. But I'm certainly not an overbearing presence!"
Eyebrows were raised by the casting of Derren Brown. How did that come about?
"I don't want to overplay it. He basically, in flashback, plays a Tudor politician who built the original house and I just thought it was a bit of fun and Derren's a friend of mine. He's a natural actor. He didn't have much to do in it but I thought it would be a nice thing for him to do and he seemed to enjoy it. I've just literally picked up a satellite listings magazine and we've got the cover, which is amazing, but it says 'Derren Brown and Mark Gatiss spook it up'. [Laughs] It's slightly overegging those two roles!"
Is it true that one of the stories was inspired by a mask you bought?
"Yes, the third story, which is about the doorknocker itself. I bought a Maori death mask in Paris a few years ago. I bought it because A Christmas Carol is my favourite story and it looked the way you would imagine Jacob Marley's face when it appeared in the doorknocker. I'm not a superstitious person, but it just felt wrong. It was hanging on the wall until one day we were woken by a large bang and I went downstairs and it was lying in the middle of the room. It was probably heavy traffic, whatever, and I thought 'well, this wants to go home'. So I rang the New Zealand Embassy and went down to see them and said 'look, I've got this thing. I don't think it should ever have left and in fact I think it might want to go home'. So I gave it to them."
Apart from the supernatural, what are the other binding themes throughout the stories?
"Well, it's an interesting question. I realised quite accidentally that it's sort of about birth, marriages and death - but the wrong way around. I suppose there's a definite thread of guilt and responsibility."
Who are your preferences to take over from David Tennant in Doctor Who?
"I obviously can't say that, so I can only really reply in the positive. It's a terribly exciting time when you're waiting for a new Doctor. I think David has done such an incredible job, to become the Tom Baker of his generation - universally loved. So it's a tall order to take over, but at the same time it's very exciting."
Has the incoming boss Steven Moffat tapped you up about writing another episode?
"Maybe, maybe. That's all I can say. The thing is, it's a way off - there's five specials before the new series so it's a way off yet. I'm writing a modern day version of Sherlock Holmes with Steven Moffat, which we're doing the pilot of next month. We're hoping it will go straight onto a series if it's liked."
Crooked House begins Monday, December 22 at 10.30pm on BBC Four.