Digital Spy caught up with Mark to chat about his new episode, his role as Mycroft and the "real scares" that await Sherlock (Benedict Cumberbatch) and John (Martin Freeman)!
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Did you feel any pressure this year after the enormous success of series one?
"To be honest, I'd have probably felt more pressure if it hadn't worked! There was a tremendous amount of good will, but… it came out of nowhere, so this year obviously we're very aware of the audience who are desperate for it, but are also feeling very proprietorial about it. But it definitely boosts your confidence more to have a hit than a flop!"
And were you nervous about tackling the most iconic stories in the Holmes canon?
"No, I don't think so. Obviously they bring with them certain amounts of baggage, particularly 'Hound' - 'Hound' is a very tough story to do, because people know it a lot more than most Sherlock Holmes stories.
"I felt I had to honour certain things, more than you would if it was just 'The Sign of Four' or something like that. But given that our whole approach to the series is from a point of view of absolute adoration, we weren't worried about tackling them - bring it on!"
So can we expect 'The Hounds of Baskerville' to be a full-blooded horror?
"Oh yes, very much. The thing is, horror is a big part of Sherlock Holmes. Doyle also wrote a lot of great horror stories, so there's a lot more horror in Holmes that people possibly think of. There's a lot of curses and mysticism and real scares.
"He wrote 'Hound', he said, to be 'a real creeper', so I wanted to maximise all those moments and basically treat it as a horror story, [though] obviously from a rational point of view.
"But I thought, 'Okay, let's really go for this'. Actually the interesting thing was to try and find, as we always do, the modern equivalent. I didn't want Baskerville Hall to be an old, spooky house, it had to be the equivalent in our age."
Sherlock remains in the background for much of the original story, but that's not the case here...
"The thing is, the best way to do it is to grab it by the throat and treat it as a joke. So [Sherlock] says, 'No, I can't come, I'm too busy - but I'm sending my best man' and then there's a reason why [he does go]. So you tip your hat to the original and then just say 'No, he's going to be in this one!'"
Next week, Moriarty's back. This show's take on the character is unusual...
"He's Irish! Though I believe Leo McKern was the first Irish Moriarty in [1975 film] The Adventure of Sherlock Holmes' Smarter Brother - I'm annoyed about that!"
It's also a more playful take on the character - how much of that was the script and how much was Andrew Scott's performance?
"The script was a big part of it. I mean, we knew we didn't want to him to be an austere older man. But Andrew came in and just blew us away. What he has is this incredible, light, playful charm, and then he turns on a sixpence and he's utterly terrifying.
"He's one of the few people I know who can really convince you he's mad, that he's psychotic, even though he's very attractive and looks inoffensive. He's not a huge, terrifying figure, but there's something terrible behind those eyes. There's something dead in him… like something else is inside him."
Mycroft also has a larger role this year...
"I know, I was delighted! When Steven said 'You're in this [first episode] a lot', I said, 'Okay… thank you!'."
Episode one showed a more emotional side to the character...
"Yes, but he still can't get involved. They're not like others, those boys! Although we've always thought that actually, rather than being a product of any kind of broken home, they're probably from a very loving family. A bit like Niles and Frasier Crane - I think there's possibly a lot of Sherlock and Mycroft in them! But there is a caring side to [Mycroft]."
You've made two series of Sherlock - are there still elements from the canon that you're desperate to adapt?
"Oh, there's millions! One of the joys about doing this show, from the start, is that for something that's a 120-odd years old and has been filmed so much, there are huge chunks of it which have never been filmed.
"The first meeting [between Sherlock and John] - which we did - has been done, I think, twice. Sherlock beating corpses to assess the extent of bruising after death - when I read that as a kid… but I don't think it's ever been done [before]!
"You come across them all the time - there are lots of famous stories, lots of less famous stories. But what we've done from the beginning is have this kind of 'magpie' approach to it, like the Basil Rathbone films. We say, 'We'll have that bit, and that bit, and that bit', [because] some of the stories have tremendous openings and very perfunctory endings."
So have you given much thought to which stories might make up a third series?
"Well, you have to see if they're going to survive this one!"
Sherlock continues this Sunday at 8.30pm on BBC One in the UK. The second series will air in May 2012 on BBC America in the US.