Its smart deductions and gripping drama have made it a global obsession. Now Cumberbatch and co are back for its darkest series yet. They explain why we’ll see a kinder Holmes this time around

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Moriarty is dead, right? But how can he be? He sent that video of himself to every TV screen in the land at the end of Sherlock’s last series, grinning to camera and repeating the words: “Did you miss me?” Fans of Steven Moffat and Mark Gatiss’s hugely popular detective thriller were left dangling as the apparently deceased arch-villain, played by Andrew Scott, taunted Benedict Cumberbatch’s be-scarfed avenger from beyond the grave. Or did he? Or didn’t he? Expect no straight answers and a barrage of curveballs as Sherlock returns on New Year’s Day.

Sherlock now has a global popularity that’s almost too intense to comprehend. Truly devoted fans follow a Twitter hashtag – #setlock – that crowd-sources information about filming locations. When the herd get wind of a location shoot, there’s a virtual stampede in the direction of Cumberbatch and co from all corners of Europe. Visiting fake Baker Street (it’s filmed on another central London street just north of Euston) on a June evening, I follow the world’s press past crash barriers, over cables, avoiding arc lights and the tripods for two giant rain machines, to see a crowd of several hundred fans on the other side of the barriers from us, waiting for a glimpse of something, anything, as the crew prepare for a night shoot for the show’s fourth series. During a take, respectful silence. Upon the arrival of one of the stars, Beatlemania.

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Laurence, 23, from Paris is wearing a deerstalker and is a member of the Sherlock Holmes Society in France. She has flown in for the day in the hope of seeing the cast and will stay all night, flying home in the morning. Her crowd neighbour is from Italy, and also flies home tomorrow. Laurence tells me: “In the group I’m in in France we have people of all ages but everybody loves Sherlock on the BBC because it’s faithful to the original but there’s a twist to it, so it’s not something we’ve seen before.” She smiles but her eyes suddenly focus on a black saloon pulling up behind me. Every eye in the now noisy crowd fixes on it as out steps Benedict Cumberbatch in full costume. He gives them a wave and canters off towards the door of 221B fake Baker Street as fans strain to keep sight of him until he’s inside.

Amanda Abbington, who plays Mary Watson, marvels at their dedication: “I’ve never spoken about Sherlock without at some point talking about the fandom,” she says. She can relate. In her teens, she was a committed Brosette. Now Matt Goss follows her on Twitter and she’s still giddy about it.

While not all of Sherlock’s millions of fans are so dedicated they’ll cross continents, you can see why he’s so loved. An actual billionaire comic-book villain is preparing to move into the White House, for real, so now seems like the perfect time for the return of this most British of superheroes. Abbington says: “If you drew [Trump] as a character in a show you’d think it wasn’t believable.” Sherlock may be a caricature of cleverness but his unashamed intellectualism is somehow reassuring as the world goes stupid. Despite the success of recent campaigns that won on a “No one likes a clever clogs” ticket, we love Sherlock because he is the cleverest clog of them all.

Facebook Twitter Pinterest Benedict Cumberbatch as Sherlock Holmes, Amanda Abbington as Mary Watson and Martin Freeman as Dr John Watson in Sherlock. Photograph: Robert Viglasky/BBC/Hartswood Films

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Co-creator Steven Moffat estimates that 150 million people globally have seen Sherlock, even more than his other hit series, Doctor Who. That’s a lot of eyes pointing at only 10 released episodes. Cumberbatch says he and the programme-makers have long puzzled over what made that first episode in 2010 catch fire the way it did. “We are slasher fiction and an updated version of a Victorian classic, so that in itself plays into the world of fan obsession,” he ponders. “Another part of it is that I’ve got curly hair and eyes that are at almost opposite sides of my head and that chimes in with a lot of people for some reason.” He is modest about his unique appearance, which has been likened to a squashed milk carton in one internet meme (he pronounces it mem), but Cumberbatch’s now frequently seen stance in a billowing coat, atop a tall building – Sherlock, Doctor Strange and in Star Trek: Into Darkness – has elevated him to one of those actors you can identify by silhouette alone.

He and co-stars Martin Freeman and Abbington stalk back on to our screens in The Six Thatchers, the first of three adventures written by Moffat and Mark Gatiss. We rejoin the crime-fighting trio with a new addition as Mary is about to give birth. Husband John is poised for new fatherhood and Sherlock is ready to apply his implacable logic to childcare. What larks. Yet, despite the cute publicity stills with a huge bloodhound and Freeman wearing a baby carrier, it all goes black as oil thereafter. The first episode centres on the curious case of The Six Thatchers, plaster busts of Maggie Thatcher that are being systematically smashed for reasons unknown. Far from a light-hearted puzzle set by an unseen playmate, the smashed Maggies point to terrible secrets from a character’s past. To be more specific would be to ruin one of the biggest “NO WAY” TV moments in years. Suffice to say, if you’re looking in one direction, Moffat and Gatiss will be setting a trap in the other.

We do know that Toby Jones, he of the large forehead and kind eyes that can switch suddenly to cold menace like a doll in a horror film, will appear in episode two as the appalling Culverton Smith, a new and dreadful nemesis for Sherlock to tangle with.

Facebook Twitter Pinterest Sherlock co-creator Mark Gatiss as Mycroft Holmes. Photograph: Colin Hutton/BBC/Hartswood Films

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Is it the darkest season yet? “I would say it probably is,” says Cumberbatch, keeping his answers short and cautious in case any plot secrets leak between his words. Is it the global doom seeping into fiction, or just a coincidence? Gatiss is still in a state of angst about the events of 2016. “I don’t know if I know this country the way I thought I did,” he frowns as we sit in the the cinema where we’ve just seen episode one of the new series. “We had a hard-won reputation for a sort of bumbly, but generally rather fair-minded thing. It’s gone out of the fucking window,” he adds sadly, referring to the post-referendum shift in the national mood.

Moffat is also despondent about events on both sides of the Atlantic. “If fiction has a role to play in this, and I’m not such a fatuous oaf that I think it really does, I think we have to start saying what being a hero constitutes.” In the new series, Cumberbatch’s detective is less of the irritatingly smug know-it-all we saw in earlier episodes. Moffat elaborates: “Being a hero isn’t being bigger, richer, more powerful than somebody else. It’s being wiser and kinder.” He pauses and adds: “I think it’s time for the less-of-a-dick Sherlock.”

Being less of a dick doesn’t mean being less of a clever dick. Those analytical fleckerls are still in evidence: thrilling flourishes of grey matter as Sherlock scans a scene and deduces a thousand impossible things about the person in front of him. Whose brain do they come from? Gatiss’s or Moffat’s? Gatiss says: “We work on them [together].” They spent some of pre-production this series wafting around in cloaks at a Moroccan retreat, meeting for meals before disappearing off to write scenes. “We have a sort of bank of them where you go: ‘Oh, I’ve got one! I’ve got one!’ If you get a good one, it’s so exciting.”

Facebook Twitter Pinterest Sherlock co-creator Steven Moffat. Photograph: David Levene/The Guardian

They harvest real-life situations, too. Gatiss was performing in Three Days In The Country at the National Theatre last year, “and one of my fellow cast members came on for the warm-up clutching 10 individual electronic cigarettes,” he says, grinning at the memory. His colleague explained that while he was trying to give up smoking, he didn’t want to commit to an electronic pipe, knowing full well he’d never completely stop if he owned one. So he bought disposable e-cigarettes in the hope that he would one day ditch them altogether. It pops up in a side-plot during The Six Thatchers.

The title is a fudge of Arthur Conan Doyle’s The Six Napoleons, a short story that featured plaster busts of the famous emperor. It’s another nod to the fans who hoover up references to the original like happy junkies. Gatiss and Moffat, both Conan Doyle fanboys since youth, know how it feels to lock on to something with such passion. Moffat says: “I have a terrifying depth of knowledge about both Doctor Who and Sherlock Holmes that makes it a miracle I’m not still a virgin.”

Freeman agrees that their Sherlock has “moments of self-reference and it’s knowing in a way. [Steven and Mark] don’t write just to please people who are fanatical about the show. But to pretend those people don’t exist would be crazy, because I’ve never known anything like the… I’m 44 but I’m saying the word fandom, fuck it – the fandom of this show.”

What about the rest of the cast? What did they obsess about in their youth? Abbington is out and proud in describing her Brossette years: “Me and my friend Jenny found out the Goss brothers used to live in Peckham and we’d look up all the Gosses and ring up and say ‘Can we speak to Matt or Luke please?’ I was completely and utterly obsessed with Matt and he was my dream man for a very long time.”

Cumberbatch says he was more of a butterfly when it came to juvenile fixations. “I had obsessions that passed with every term at school, whether it was skateboarding, or Harrison Ford or Transformers.” But he was partial to The A Team. When pressed on which member he’d like to be he admits, “After a coffee, probably Murdoch and if I was at my red-carpet cheesiest then definitely The Face. When I’m savvy enough to get my shit together and organise things, I’d like to be Hannibal.” He seldom gives an unequivocal answer when talking about himself. But when his every twitch and raised eyebrow finds its way online, it’s no wonder he employs caution. Despite its intensity, the fan presence is viewed as benign and respectful by cast and crew.

Facebook Twitter Pinterest Cumberbatch and Freeman as Holmes and Watson. Photograph: Todd Antony/BBC/Hartswood Films

The level of obsession is nothing new where Sherlock is concerned. In an article in the Strand Magazine in 1917, Conan Doyle recalls an encounter with a fan: a Cornish boatman who’d recognised the writer while on holiday. Gatiss takes up the tale, doing the voices. “He said: ‘You wrote Sherlock ’Olmes?’ And he goes, ‘Yes.’ ‘Right, I like them.’ ‘Thank you.’ ‘Mind, ’e were never the same when ’e came back from the dead.’ He was, like, a troll! The first one,” hoots Gatiss. One minute they’re attacking you with an umbrella (as one woman was said to have done to Doyle) for killing off their hero, the next they’re complaining about the way you brought him back.

Moffat and Gatiss were similarly scalded when their hero returned from the dead at the start of series three. The title of the last episode in this new series is The Final Problem, the same as the story in which Conan Doyle originally sent Holmes to his (temporary) doom. Read into that what you may, but the writers are having a whale of a time keeping us all guessing. “Do you lie in interviews?” they’re asked at a press junket. “Yes,” grins Moffat. “No,” says Gatiss, grinning even more broadly.

Whatever the fans are expecting next, their ardour is sure to burn as bright as ever for the new series. Not just because they fancy Cumberbatch or get weak knees over that flaunting of mental dexterity. But because the idea of a wiser, kinder hero who provides order to the chaos, is worth hanging on to right now.

Series four of Sherlock starts on New Year’s Day, 8.30pm, BBC1