Sherlock Holmes: Benedict Cumberbatch and Martin Freeman were originally unsure about detective's Victorian adventure

Having so successfully updated the detective stories for the BBC series, its creators wrote a special episode in which Holmes and Watson are seen in their original era. But its stars Benedict Cumberbatch and Martin Freeman explain that they had to be convinced

Benedict Cumberbatch with Martin Freeman as John Watson (BBC)

Benedict Cumberbatch fears he might have a reputation for being “a dick”. This endearing glimpse of a major star self-aware enough to see beyond the legions of so-called “Cumberbitches” and other worshipping admirers comes during a discussion of the coat that he wears in BBC1's Sherlock. For a while, it seems, Cumberbatch used to wear Sherlock's coat off-set as well, after Sherlock co-creator Mark Gatiss told him that the garment suited him.

“But then I started to get a bit self-conscious about being photographed,” he says. “I might be seen wandering out and about wearing his costume and seal my reputation as being a dick.” The coat – now tatty – is safely back in storage because Cumberbatch's Sherlock is currently looking a lot suaver and smarter than the rather scruffy, overgrown undergraduate to whom we have become accustomed.

The upcoming Sherlock special, The Abominable Bride, takes place in Victorian times and his character's normally unruly mop is slicked neatly back, and Cumberbatch is wearing Victorian evening wear. He looks more suited to a Pall Mall gentlemen's club than to an unheated rehearsal room at the production's home in a former bottling plant in Bristol.

Ageless: Benedict Cumberbatch as a Victorian Sherlock, on the set of the special episode (Robert Viglasky)

“I was thrilled,” he says of playing Sherlock in his original 1890s form. “At last, I could get a haircut.” The snip, it transpires, is also something of a metaphor for the relief of setting Steven Moffat and Gatiss's updated detective back in his original era. “You feel some of the weight is taken off you,” Cumberbatch says. “You're no longer trying to establish this man in the 21st century. The other gorgeous thing about going back in time is that you can actually look to the books for source material, which I always do for our version anyway, but it's even more qualifiable to lean on them for inspiration.”

Not that The Abominable Bride, unlike earlier episodes of BBC1's triumphant series, is based on an actual Sir Arthur Conan Doyle tale. Its inspiration is a case mentioned in passing by Dr Watson in the 1893 short story, The Adventure of the Musgrave Ritual – a mystery that Holmes had solved before his acquaintance with Watson.

On set with Sherlock (BBC)

Cumberbatch claims that he wasn't initially convinced by Moffat and Gatiss's idea of a standalone episode transporting his painstakingly modernised detective back to 1895. “In fact, I went, 'You're mad',” he says. “I genuinely didn't understand how they were going to get away with it.

“And then I got the more detailed pitch and I thought, 'OK, this is going to be great fun', And it really is. It's so nice to play him in his era. The things that are asked of me in the modern version, the sense that this is a man clearly slightly out of his time... to put him back in the era he was written in originally is just a joy. It feels easier.

“And then there are things I tried to impose on the modern version, like his stature and physicality – a lot of that's done [in the Victorian version] by the clothing, the collars, the deerstalker and cape and pipe and things.”

Ah yes, the deerstalker, cape and pipe. Didn't he feel a bit of a walking cliché when armed with the detective's iconic accessories? “And yet it doesn't feel like a cliché because you're functioning in them rather than quoting them,” Cumberbatch says. “They were de rigueur items of fashion that have just become iconic for him, but they're very useful. And there might be a magnifying glass that might be slightly bigger than the one I usually use...”

It has been nearly two years since the last full series of Sherlock, the final episode ending with a cliffhanger, that of a video close-up of Jim Moriarty's face being broadcast all over London, asking: “Did you miss me?” Andrew Scott's arch villain doesn't feature in the Victorian special, which will be simultaneously screened in cinemas across the UK and across the world (including China, where Sherlock has a huge following). A full new series starts filming in the spring.

“We are very good at making people wait – it's what we do,” quips Gatiss. “Whole civilisations have risen and fallen between seasons of Sherlock.” As they have established Sherlock Holmes and John Watson so brilliantly in the 21st century, I wondered why Gatiss and Steven Moffat wanted to now place them in a Victorian setting?

“It's called the Adventure of Having Your Cake and Eating It,” says Gatiss. “No, to be honest, it was just too irresistible to see Benedict and Martin [Freeman] and everyone else in Conan Doyle-land. Given that it's fair to say that Benedict and Martin are the Holmes and Watson of their age, wouldn't it be awful if you never saw them do it properly? We sort of joked about the idea for a long time; the only other people who have done both period and modern Holmes and Watson are Basil Rathbone and Nigel Bruce, who started Victorian but eventually fought the Nazis in the 1940s.”

Moffat adds: “It started with us seeing if we could justify doing a 10-minute version where they put the togs on so we could see them do it. And we thought of all the jokes we could do, and then we thought, 'Actually let's not do that, let's do it properly, not tongue-in-cheek'.”

“But it's still our show, it's still essentially Sherlock,” says Gatiss. “It hasn't suddenly become very dusty and slow. We knew that we didn't just want to do a Comic Relief sketch. This is a full-blooded Victorian Gothic.”

For Freeman, one of his biggest objections to the Victorian setting is the bushy moustache he has to wear on his top lip. “I'm going to try to rein in that in series four,” he says, “and not let Steven and Mark think this is an ongoing thing now, or I'll end up as Robinson Crusoe.”

Like Cumberbatch, Freeman was also initially resistant to the idea of a Victorian episode. “But then I was originally resistant to Sherlock because it was modern,” he reveals. “Before I read the scripts [for series one] I thought 'Hmmm, modern Sherlock Holmes could be rubbish. I've overheard Mark and Steven say a couple of times while we've been on set that, 'Finally we're doing it properly, we're doing the correct version at last'. It's nice to ring some changes, I guess.”

Indeed, for those who might have been hoping for a business-as-usual, modern-dress Sherlock, Freeman has this to say: “I believe in not just giving people what they want because why should you? I mean, there was resistance about series three among diehard fans [some thought it had become too introspective and lost its storytelling mojo], but give them a couple of months and they watch it again. It sinks in.”

After three years spent in New Zealand playing Bilbo Baggins in Peter Jackson's Hobbit trilogy, as well as six months in Canada filming the first season of Fargo, Freeman is enjoying a relatively relaxed 2015. Filming Sherlock means an opportunity to work with his wife, Amanda Abbington, who has played John Watson's wife, Mary Watson, since the start of series three. He is, however, resigned to the cost to his family life of his profession.

“There's no way round it,” Freeman says. “Well, there is; I could stop acting, and even though it's second in importance to my family, it's a close second because I was doing it before I met Amanda, and I was doing it before I became a dad.”

The 44-year-old Freeman's upcoming workload includes Funny Cow, starring Maxine Peake as a stand-up comedian trying to make it in the macho world of northern working men's clubs in the 1970s and 80s, and being reunited with his Fargo co-star Billy Bob Thornton (as well as playing Tina Fey's Scottish boyfriend) in the war comedy Fun House. Will there always be room for Sherlock?

“We all know it's a good show, but the truth is that it has got more and more difficult to factor in,” Freeman says. “I think I will, though, as long as we're all free and enjoying it. I've always believed in doing things as long as one wants to do them, and as soon as you don't want to do something I think you should stop. Unless it's marriage – and then you should work on it...”

Cumberbatch, who will turn 40 next July, is understandably one of the busiest actors around, currently filming the Andy Serkis-directed Jungle Book: Origins (he plays Shere Khan) opposite Christian Bale and Cate Blanchett.

He also takes the title role in Doctor Strange, the latest Marvel Comics blockbuster and – following his Oscar-nominated role as Alan Turing in The Imitation Game, he is preparing to portray Thomas Edison in The Current War, which tells of Edison's struggle with George Westinghouse (played by Jake Gyllenhaal) for control of the nascent electricity market in the 1880s. With so much going on, how determined is he to keep making time for Sherlock?

“Pretty determined,” says Cumberbatch. “I'm still enjoying it. We'll see how the next series goes, but I'd love to keep ageing with him. It would be an interesting experiment to do. Martin and I started this relatively young compared to other Holmes and Watsons, so why not?”

'Sherlock: The Abominable Bride' will be screened on BBC1 on New Year's Day, and in selected cinemas worldwide