Benedict Cumberbatch, Martin Freeman on the eagerly awaited return of Sherlock

All-conquering Sherlock returns to our television screens on New Year’s Day. In a regional exclusive, stars Benedict Cumberbatch, Martin Freeman and Mark Gatiss tell us why the show is more popular than ever.

Benedict Cumberbatch returns as Sherlock Holmes in series 4 of Sherlock. Courtesy Todd Antony / Hartswood Films / BBC.

Sherlock Holmes looks terrible. Tired, dishevelled and haunted, he shakes as his brother Mycroft says: “The roads we walk have demons beneath. And yours have been waiting a very long time.”

Meanwhile, supposedly-dead arch-villain Moriarty repeats: “Did you miss me?” over and over again.

And this is just the trailer for Sherlock season 4, which begins tonight on BBC First, available on OSN.

With Sherlock boasting all the intrigue, action and suspense of the very best Hollywood blockbuster – indeed, the final episode of the show, which made a star of Benedict Cumberbatch, will be screened worldwide in selected cinemas – it is no exaggeration to suggest it is the most eagerly awaited television show of 2017.

Thankfully, Cumberbatch is looking much more chipper than his character when we meet him during the filming of season 4. His sunny disposition is probably thanks to the huge challenges – and rewards – Sherlock is presenting the Oscar-nominated actor with.

"I love the show more, and Holmes more, because of how I've been able to experiment with who he is and what he does," he says. "And in this series he is troubled and challenged. At the top of his game and at the very bottom of his soul. There are huge highs and huge lows, and you really find out who he is. It's the Holmes you know, and the Holmes you don't know."

As far as plot details go, that is all we really find out during our day on the Sherlock set, talking to the impressively tight-lipped cast members, producers and writers.

John (Martin Freeman) and Mary Watson (Amanda Abbington) now have had a baby – co-creator, co-writer (and Mycroft actor) Mark Gatiss says it "changes everything" – but that is as much as we get. We do not even learn of the casting of Toby Jones (Captain America, The Hunger Games) as the evil, almost Joker-like Culverton Smith until afterwards.

But then, that's the joy of Sherlock: its huge international success (it has sold more DVDs in China than Star Wars) is because it is genuine event television, where the audience sits down with Holmes and Watson for three 90-minute episodes each season, and is simultaneously wowed and confounded by Holmes's sleuthing skills. Spoilers would ruin all the fun.

“We aimed high to begin with and we keep on trying to surprise ourselves and our audience,” says Cumberbatch. “The level of invention, imagination, darkness and joy is beautiful – it’s bleak at times but it turns on a dime into something very funny as well.”

There’s also a sense of a journey. Over the past seven years – three seasons plus a Christmas special – it has been fascinating watching Holmes’s character gradually evolve.

As Cumberbatch puts it, he has “slowly thawed from someone locked in glacial ice into someone who we can relate to and understand”.

In the new season, that means Holmes is no longer the invincible hero. He makes some “spectacular own goals and mistakes”, but always has Watson to fall back on. Their chemistry is as intriguing as the crimes they try to solve.

“They’re an odd fit – the one crossover is danger, the thrill of the chase, the excitement of being in a world less ordinary,” says Cumberbatch. “That’s the risk that both of them take, with terrible consequences at times. But it draws them together, and in this series it’s become very clear that Watson is a humanising influence on Holmes.”

Freeman does not have a problem with playing Holmes’s intellectual inferior, because he says: “It’s generally accepted that Sherlock is the brains and John is the heart of the show.”

And though Watson has become as important on Freeman's CV as The Office, Fargo and The Hobbit, it is fascinating to hear that initially he had misgivings about the whole premise of the show.

“My reservations were that it would be too cool and knowing in the modern-day setting,” he says. “But as soon as I read a few pages, I knew it was smart, exciting, thrilling. The writers allow you to do your best work, and I think we’ve gone far enough away from Arthur Conan Doyle for it to be our show – but not so far that the fabric doesn’t hold.”

Gatiss adds: “Our abiding principle, right from the beginning, was to get back to what Conan Doyle did.

“The deductions and adventures are all part of it, but it’s principally Holmes and Watson’s relationship, and we wanted to restore that and give it our own spin. The devotion that has provoked from people around the world has been astonishing – and a great joy.”

Co-creator Steven Moffat, who is also the showrunner on Doctor Who, another huge BBC hit, shakes his head in mock astonishment as he considers the success of the show and its stars. "Benedict Cumberbatch," he says. "He went from 'are you making that name up?' to the most famous living Englishman in the space of 90 minutes one Sunday."

There is a precedent for this: Conan Doyle was baffled by the success of his stories in the late 19th century. So why do people love the TV adaptation so much?

"Well, the literature itself is so good," Cumberbatch says. "To all intents and purposes, our version is slasher fan fiction, which people love. Holmes is also an outsider who speaks to a huge teen audience, but he crosses generations, traverses cultures, ethnicities, national divides. But I'm still learning things about Sherlock and I think we all will in this new series – devastatingly so."

Later, we find out that the last episode in this season is called The Final Problem. You have been warned…

• Sherlock season 4 begins tonight on BBC First, OSN