Best known for his role as Sherlock's nemesis Moriarty, Andrew Scott turns spy for the BBC's new one-off Cold War drama, Legacy.
Here he tells us about being Russian, and answers those burning Sherlock questions...
What can you tell us about Viktor Koslov, the character you play in Legacy?
"Viktor's a sad, lonely character, who's made a lot of sacrifices for his job, so it's important we don't just see him as a typical baddie. What makes Legacy different to other spy dramas is that it explores how being a spy affects a person as a human being and impacts on their family life."
The drama centres on the exploits of Charles Thoroughgood (Boardwalk Empire's Charlie Cox), a trainee spy with MI6 who's asked to revive his friendship with his old university pal, Viktor, to uncover information on a possible attack on the UK. How does this affect their friendship?
"As a spy, you have to tread on people to obtain information and Charles and Viktor's friendship is certainly jeopardised because of the jobs they both have to do. There's a real personal cost to Charles and Viktor and, even though they have to outsmart each other, deep down, nobody wins."
How did you find having to master a Russian accent for the role?
"I had a really good dialect coach who helped me with it and I watched lots of YouTube clips of Vladimir Putin. I just desperately didn't want to sound like Count Dracula!"
Was the Cold War period of the early Seventies something you knew much about or had much interest in?
"Yeah, it's kind of been represented quite a lot on film, hasn't it, and there's something terribly atmospheric about the Seventies that lends itself to spy dramas. Back then, they didn't have the mobile phones and technology that we have now, they relied a huge amount on their wits and human instincts."
The drama looks very authentic - was it ever tricky trying to create that Seventies' feel during filming?
"Yes, it could sometimes be a nightmare to shoot. We filmed some scenes on the hottest day of the year on Hampstead Heath, so we had to block out people with iPhones and luminous green acid house jumpers."
Would you say Viktor is as dark a character as Sherlock's Moriarty?
"I never see Moriarty as a dark character. Of course he's the personification of evil but, actually, he's great fun to play and there are a lot of jokes there. I wouldn't say I'm attracted to dark stuff - it's just always good to be part of a great story and play a really interesting character."
Could you have predicted the level of recognition that Moriarty would give you?
"Absolutely not, no. I don't think any of us had any clue as to how successful it was going to be; how instantly affected people would feel about it. As Moriarty is essentially playing lots of different characters within a character, it's allowed me to play other characters with a bit more ease."
Sherlock creator Stephen Moffat says that Moriarty is definitely dead, but may appear in the next series in flashback...
"Ha, ha! Well, of course, you'd have to trust everything Stephen Moffat says! When people ask me about Moriarty, I just have to nod and smile."
How do you think you would fare as a spy?
"I think I'd be quite a good spy actually. I think to be a good spy you've got to be good at listening to people - and my family and friends say I'm a good listener, so maybe that's what it is."
You've managed to keep the Sherlock secret...
"Well, exactly! I seem to spend every day of my life keeping secrets!"
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