The case of the amazing reinvention: On set with the stars of TV’s Sherlock

Next Friday sees the nationwide cinema release of Sherlock Holmes: A Game of Shadows, the follow-up to Guy Ritchie's 2009 blockbuster starring Robert Downey Jr as Holmes and Jude Law as Watson – a cavalier re-imagining of Sir Arthur Conan Doyle's fictional detective for the age of the multiplex. Viewers willing to accept fiction's greatest violin-playing sleuth as an unkempt, prize-bareknuckle-fighting slob and the stolid Watson as more kick-arse than Jason Bourne have presumably never been anywhere near the source novels – or if they have, they will be less proprietorial, and have a greater taste for absurdity, than your average Holmesian purist.

These aficionados need only hang on until the New Year, however, for the return of Sherlock, BBC1's witty and deeply knowledgeable contemporary reworking of the stories, and an overnight sensation when first screened last year. The three new 90-minute dramas, once again starring Benedict Cumberbatch and Martin Freeman, may be set in the here and now, but they are in 100 ways more true to the novels than Ritchie's lavish Victoriana. Hell, they've even got the thumbs-up from keepers of the sacred flame, The Sherlock Holmes Society. The show's co-creator, Mark Gatiss addressed its members before the first series broadcast.

"You can imagine that there wouldn't be any more ossified or reactionary group of fans and they all loved it," he tells me. "In fact we had some people come to the screening of the first episode and one of them said he thought it was the best on-screen depiction he'd ever seen."

In the company of Gatiss (who also plays Holmes' brother Mycroft) and his Sherlock co-creator Steven Moffat, I'm snooping around the living room of 221B Baker Street – not the real one, of course, because that never existed, at least not in Conan Doyle's day. When Baker Street was, later, extended, the address was subsumed into the Art Deco Abbey National Building Society HQ (it even employed a secretary to answer fan mail), and has since been appropriated by the Sherlock Holmes Museum, which is actually situated between 237 and 241 Baker Street.

This particular 221B – a bachelor-pad clutter of books, empty bottles and crisp packets alongside test-tubes, a chess set and a music stand ("We had to be careful – we didn't want it to be too Men Behaving Badly," says Moffat) has been constructed in a warehouse on an industrial estate on the outskirts of Cardiff. It's right next door to where Doctor Who is filmed, in fact, and the connection between the two shows is far from coincidental, it being on the train between London and the Welsh capital that Doctor Who writers Gatiss and Moffat started discussing their love of Holmes, dreaming of a modern-day version, and sharing their esteem for the 1940s movies starring Basil Rathbone and Nigel Bruce. Holmesians seem to prefer the 1980s ITV shows starring Jeremy Brett.

"Brett's Sherlock, like all the great Sherlocks, is not quite the original," says Gatiss, a man so steeped in his subject that you wouldn't be surprised if he wore a deerstalker in the privacy of his own home.

"He's a much madder, more manic creature. But, anyway, he resurrected it from being something that was about to expire." Interestingly, there have been 20-year gaps between resurrections, says Moffat. Rathbone and Bruce in the 1940s were followed by Peter Cushing in the Hammer Holmes films of the Sixties (by coincidence or not, Cushing was also the cinema's Doctor Who), and Jeremy Brett 20 years later. And now, after another two-decade hiatus, we have the simultaneous but very different Downey Jr and Cumberbatch incarnations.

"We were actually making the pilot when we heard [about] the Robert Downey film. It just happens; there's always two Robin Hood films at the same time. I do think we're diametrically opposite – almost the two extremes of what you can do with it. They co-exist happily," says Moffat.

"When I saw the trailer [for the Downey Jr film], I didn't expect to like it" says Gatiss. "This is too radical for me... this is just turning him into an action hero. But then I saw it – Sherlock redone as Hollywood blockbuster, but Sherlock Holmes redone as a Hollywood blockbuster really well – I loved it." I'm afraid I can't share his enthusiasm, but then I'm not mad for Hollywood action films at the best of times, and any love shown to Guy Ritchie's movie was as nothing compared to the adoration showered on the BBC1 Sherlock. Screened during the summer holidays – a season traditionally treated as a kiss of death for new shows by less imaginative TV schedulers – the show was an overnight success, trending on Twitter, debuting with nine million viewers, and being praised by Culture Secretary Jeremy Hunt in the House of Commons (albeit during an otherwise negative assessment of the number of repeats on the BBC).

"I had a certain amount of trepidation," says Gatiss. "But we were always very confident [that] as soon as you saw a couple of minutes you'd get it. However the extent to which people got it took us all by surprise – within the first episode we seemed to have developed an enormous fanbase." And that fanbase is worldwide, Sherlock having been sold to 180 different countries (Downton Abbey sells to about 100), viewers going particularly crazy for it in Russia and South Korea. Indeed Cumberbatch started noticing an unusually international make-up to the audiences queuing to watch him last year in Terence Rattigan's After the Dance at the National, and foreign fans would stop him on his way home.

"I had done a long day and I said, 'I don't wish to be rude...' and they said 'we've come all the way from South Korea'... People came from Russia, Japan, everywhere." Freeman, for his part, says he is now on the radar of the under-20s – teenagers too young, amazing as it may seem, to remember him in The Office.

When Twitter exploded into appreciation after the end credits of the opening episode, "A Study in Pink", Cumberbatch said that he half expected "people abseiling down into our garden just to get a sneak peek at us. We silenced a lot of the doubters – I'm a bit sick of hearing, 'Oh God I wanted to hate it so much but I was surprised how much I loved it'. Knives were sharpened... I think a lot of people thought 'that'll never work'."

What people thought would never work, of course, was updating the Sherlock Holmes stories to the 21st century, with Sherlock as a freelance police adviser (er, try getting that past the Police Federation) and John (Watson), an army doctor newly returned from Helmand province. Despite the talent involved, I was one of those doubters – mainly because I believed that the Sherlock Holmes school of forensic crime-solving, albeit pioneering, had been copied to death, most recently by the CSI franchise.

"There's a line in this new series about CSI Baker Street," says Moffat. "But the gathering of information isn't critical to Sherlock – it's what he can conclude from it." And Gatiss says they did ask themselves whether Holmes could survive nowadays. "The key really is that he's still the cleverest man in the room," he says. "Of course the whole notion of forensics came from Sherlock Holmes and there is a team of people in SoCO [Scene of Crime Officers] suits... but he is the one [who asks], 'but did you see the hole in his left thumb, or 'did you notice he was limping with the other foot?'"

Sticklers for procedure may, by the way, be wondering why Holmes doesn't wear evidence-protecting clothes when visiting a crime scene. "In the pilot we did put Sherlock in a SoCO suit and we didn't do it again," says Gatiss. "You just have to bend rules because he has to have his coat on because he looks ridiculous without one."

This anomaly apart, Gatiss and Moffat embraced what they call "equivalence" – finding modern equivalents to dated details. After all, only the most affected young person would smoke a pipe these days, so this Holmes is trying to kick a cigarette habit with nicotine patches (there is no cocaine habit in Sherlock, but then the whole subject is a lot more thorny than it was in freely-snorting mid-Seventies Hollywood, when Herbert Ross made The Seven-Per-Cent Solution, the title of Ross's film taken from the make-up of the detective's cocaine tincture). Also, the act of regular letter-writing is nearly extinct, replaced by texts and emails, and one of the happy innovations of the first series was the use of clouds of words on screen so viewers didn't have to endure endless cutaways to mobile phones.

But for both Gatiss and Moffat, the chief reason for Sherlock's success can be found on a more human level, in the relationship between Holmes and Watson – a friendship turned into a Butch Cassidy and the Sundance Kid-style bro-mance in the Guy Ritchie film.

Cumberbatch and Freeman are beautifully cast, their characters correlating with the personalities of the actors (Freeman laid back and down to earth; Cumberbatch more garrulous and intellectual), although the latter is quick (boy, is he quick) to distance himself from Holmes. "Because I talk a lot... probably because I'm nervous and can't quite edit the thoughts into something a bit more pithy... I get pinned into the same mania bracket, or having the same level of energy... I really don't. I'm very lazy in comparison to Holmes and I operate at a far lower speed."

Martin Freeman thinks his real-life relationship with Cumberbatch is reflected on the show. "And that's not lost on the writers," he says. "We didn't know each other before so all of our relationship comes from Sherlock." Not for much longer, however, as shortly after we meet, both actors are due to fly to New Zealand to appear in Peter Jackson's movie of Tolkien's The Hobbit (Freeman as Bilbo Baggins; Cumberbatch as Smaug). Jackson, it transpires, is a big fan of Sherlock, as is Steven Spielberg. "Spielberg came down on set," says Cumberbatch. "He went nuts for it; he was very, very nice about it."

The actor reveals that, although he loves the role of this (in Holmes's words) "high-functioning sociopath", especially being rude to people, it wasn't easy returning to it. "When I went back it felt like a pale impression," he says. "It felt like something I had seen on the telly last year. Re-engaging it with it physically... everything... just felt a little bit odd." That other celebrated Holmes, Jeremy Brett, was, of course, nearly consumed by the role.

The new trilogy of 90-minute films begins with "A Scandal in Belgravia" (an adaptation of Conan Doyle's first Holmes short story, "A Scandal in Bohemia"), followed by The Hound of the Baskervilles (here "The Hounds of Baskerville"), and is rounded off by "The Reichenbach Fall", a version of "The Final Problem", the 1893 story in which Holmes seemingly tumbles to his death during a fight on an Alpine ledge (this story also forms part of the new Guy Ritchie film).

"We're doing the biggies," says Moffat. "In a funny kind of way it looks like we're racing to get it finished or something but it's not true, it's about telling an epic story."

As anyone who knows their Holmes will tell you, the chronology here is skewed, because The Hound of the Baskervilles is the Sherlock novel that Conan Doyle wrote after the hiatus of eight years, having intended to kill off Holmes at Reichenbach Falls. But then Gatiss and Moffat are mashing the whole canon.

"Almost all of the stories are not long enough," says Moffat, "We did "The Great Game" last year and that's adapted from about 28 Sherlock Holmes stories."

One of the briefest of the short stories is "A Scandal in Bohemia", in which the about-to-be-married King of Bohemia is being blackmailed by a former lover, Irene Adler, the only woman ever to impress the great detective, after she gets the better of him. Or, as Watson recalls at the start of the original story: "To Sherlock Holmes she is always THE woman... in his eyes she eclipses and predominates the whole of her sex... and that woman was the late Irene Adler, of dubious and questionable memory." Rachel McAdams portrays Adler in the Guy Ritchie movies, while, in the new BBC film, the part is played by Lara Pulver – Erin Watts in the final series of Spooks – and Moffat is rightly being coy as to how badly Cumberbatch's Holmes will fall for her.

"I do know the original story well and you have to respect the limits of it," he says. The most widely known and oft-filmed Holmes story of them all, however, is The Hound of the Baskervilles – the novel written in 1901 after Conan Doyle returned from the Boer War, resurrecting (for financial reasons, it is thought) his character from his watery grave in Switzerland – although it is Watson who, famously, does most of the Dartmoor legwork.

"The big problem is that the dog is always disappointing," says Gatiss. "But what it is is a story drenched in atmosphere. It's the closest Sherlock Holmes gets to a horror... it's a ghost story... and what we realised is that you sit in a room and feel the weight... because so many people have sat [in a room like this] saying 'How are we going to do the dog?'."

In the editing suite I am shown the first 15 minutes of "The Hounds of Baskerville", and, without giving too much away it involves Russell Tovey (Him & Her, Being Human), a Porton Down-style chemical weapons and research facility, and an escaped mutant animal. "It's trying to find a way of making a monster believable now," says Gatiss. Whatever difficulties they have experienced with the plausibility of the three new stories, the biggest hurdle has now long been traversed, and that is getting audiences to accept a modern-day Holmes in the first place.

"The thing about the updating is that last year we thought about it all the time," says Moffat. "Now I go for days without thinking this is any different from any other Sherlock Holmes." But did they ever consider retaining the Victorian setting?

"At the very last minute... I remember thinking, 'Is this just a thought-experiment? Should we be pitching it as a Victorian version?' And then we thought, 'no, let's do the modern version because that's cooler'. If you can just get past the first and only heresy, the rest of it is remarkably faithful."

'Sherlock' returns to BBC1 in the new year. 'Sherlock Holmes: A Game of Shadows' is on release next week