The genius crime-solver (Benedict Cumberbatch) finds himself on uncertain ground when he encounters confident, powerful dominatrix Irene Adler (Lara Pulver).
After a screening of the phenomenally good series premiere, Digital Spy caught up with the charming Miss Pulver to discuss the modern-day Adler, her infatuation with Sherlock and how it felt to lounge about in the nude!
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The modern Irene is a dominatrix - how was it filming the saucier scenes?
"It was a very different challenge to what I've ever faced before as an actress. Just being intensely vulnerable, I guess. Once you get over that, it was kind of empowering in a bizarre way.
"You can't hide behind anything - there's no mask. This is me, in the flesh! By the end of take two, it wasn't an issue. Onwards!"
How did you come to join the Sherlock cast?
"I'd just wrapped on Spooks and I was reading the script for Sherlock on a plane back to Los Angeles. I just thought, 'Turn the plane back around'! I put myself down on tape and they responded, so I flew back, met Steven [Moffat] and read with Benedict. I got the role and then we started rehearsals two days later.
Irene must have been a coveted role - were you worried about the competition?
"D'you know, it didn't even occur to me! I was so taken by the material that I just thought, 'I want to play this - I want to have this challenge'."
Were you nervous about joining the show after its huge success last year?
"For me, it was the best thing last year on TV. Series one was just the best thing I'd seen. I walked in and they'd just won BAFTAs and been nominated for Emmys, and it was just a massive cauldron of talent.
"On the creative side, across the board, it was so exciting - they had this wonderful air of confidence off of the back of series one. And I felt very privileged to be invited into that world."
Did you research the character of Irene Adler when you got the part?
"Well, Steven had written such a multi-dimensional role that I literally immersed myself in his writing. Everything else went out the window for me, research-wise, other than [the dominatrix angle]. Literally, if you search my computer, you'd think I was a porn star or something! So that was my main research - sex, sex and more sex!"
How would you define Sherlock and Irene's relationship?
"It's like they're looking in a mirror at each other. They're just in tune - they get each other. They see each other and I think they see through each other's masks.
"Irene wears her mask so well - this powerful dominatrix - and he sees through it. And she does [the same] with him. It's an infatuation. A mutual infatuation."
Assuming Irene survives 'A Scandal In Belgravia', would you like to return to Sherlock in the future?
"It would be a privilege to work with Steven, Mark and the entire team [again] on this project, and on other projects."
You have a terrific scene with Martin Freeman (John Watson) in the episode...
"It was an acting lesson - an acting workshop! Martin Freeman is a genius, he really is. He gives you every colour of the rainbow in every take and it's wonderful just to play off of him and opposite him."
Are you prepared for the reaction of dedicated Sherlock fans?
"I'm sure lovers of Benedict will not approve of my naked lounging - I'm sure there'll be complaints there! But hey, if they love him and find him charming, I'm with them - I'm a fan!
"But people are just hugely supportive - they love the show. And I'm sure it will continue, because the work that [director] Paul McGuigan and the entire team has put together is extraordinary."
Sherlock returns to BBC One on January 1 at 8.10pm.