Sherlock series 3: Benedict Cumberbatch and Martin Freeman provide teasers for the biggest comeback in British television

“You’ll have to wait and see,” says Cumberbatch, drumming his fingers on the table. “I know it’s frustrating, this game, but you won’t be disappointed.” I had been fishing for clues about THE question obsessing fans since “The Reichenbach Fall” ended with Sherlock throwing himself from the roof of St Bart’s Hospital in London.

How does he survive? For obviously he does survive – as Freeman says, “It’s not just going to be me... it’s not called John yet.” And, anyway, Holmes was seen in that final graveyard shot, watching Freeman’s Watson making his touching soliloquy (“I was so alone and I owe you so much” – it could be an epitaph for the ultimate Sherlock geek).

“I was as curious as the nation was to figure it out. I had my own idea and it wasn’t far off,” says Cumberbatch with a Sherlock-like lack of false modesty. “It was as surprising and delightful when I read about it in the script as I hope the audience will find it.”

So was the falling body really Moriarty strapped into a Sherlock mask? Or did smitten pathologist Molly supply a corpse to be thrown from the roof? Or does Sherlock’s Byronic greatcoat obscure some sort of base-jumping parachute? “There is a clue everyone has missed,” co-creator Steven Moffat has said. “So many people theorising about Sherlock’s death online – and they missed it!”

Freeman reckons not unreasonably that most viewers actually prefer surprises to spoilers. “However much people say, ‘Oh, go on tell us’, they wouldn’t thank you for it once the show goes out,” he says. And Gatiss, who had to conjure up the solution to this cliff-hanger, adds that we shouldn’t obsess too much about it anyway. “It’s the resolution without the resolution becoming all that it’s about,” he says of the opening episode, “The Empty Hearse”, which takes its title from the short story in which Sir Arthur Conan Doyle resurrected the Sherlock Holmes he had tried to kill off 10 years earlier.

“You can’t spend 90 minutes explaining how he did it,” says Gatiss. “Everybody’s very excited about it now but I guarantee everybody will forget about it as soon as it’s done because for us [the episode] was about restoring the friendship between John and Sherlock.”

The black hearse revealing Sherlock's return

In the Conan Doyle tale, Holmes returns to London disguised as an elderly book-dealer. Watson faints when his old friend reveals himself, but after that it’s business as usual. Gatiss says that there won’t be any disguises in the Conan Doyle sense: “For all the potential fun of putting on putty noses or ginger wigs, it’s actually quite spooky if you go, ‘Oh he was here all the time… he was hiding in plain sight’.” He was keen for a more emotionally challenging reunion. No Victorian stiff-upper lips here – this will be more the 21st-century variety.

“The dynamic shifts, which is really exciting,” says Cumberbatch. “There’s a bit of explanation and it’s not like the stories where Watson goes, ‘Oh, good… what’s the next case?’” Indeed, like many a close bachelor male friendship, this one is threatened by a woman.

The clues? Firstly, there is going to be a wedding in the second episode, “The Sign of Three” (as producer Sue Vertue, Moffat’s wife, has relayed via Twitter), and in the original stories Dr Watson does indeed settle down with one Mary Morstan, heroine of the novel The Sign of Four. Moreover, Morstan is being played by the actor Amanda Abbington, Freeman’s real-life partner, who described  herself recently as “a kind of third wheel” who will come between Holmes and Watson.

“Mark and Sue had worked with Amanda before and they contacted her,” says Freeman, obviously sensitive to accusations of whatever is the uxorious equivalent of nepotism. He won’t be drawn on their on-screen relationship. “John’s circumstances have changed in a way that you will see. He has to face the fact – as he sees it – that his friend has died. So he’s trying to have a reasonably steady and stable life.” Sounds like marriage, then? “We do have some scenes together, yeah, but she [Amanda] will be involved in some scenes I’m not in.”

So much for the potential love interest, but what about the villains now that Moriarty has blown out his own brains? Or did he? In a recent interview with this newspaper the actor Andrew Scott, who played Moriarty, did not exactly rule out the possibility of a Lazarus-like return, although Freeman, in what could possibly be a case of  double bluff, tells me: “Unless Moriarty did  something incredibly Derren Brown-like we’d have to assume there is no more Moriarty.”

Being Derren Brown-like would certainly not be beyond Moriarty. But, sticking to what we do know, the new season’s third episode, “His Last Vow”, features the Danish actor Lars Mikkelsen (Copenhagen politician Troels Hartmann in the first series of The Killing, and currently in Borgen) as Charles Augustus Magnussen, who is based on Conan Doyle’s character Charles Augustus Milverton, “the king of all blackmailers”. “We will present you with new villains”, says Gatiss cagily. “The point to Moriarty was that he was the  baddest – he was the anti-Sherlock Holmes – and you don’t want to provide a lukewarm version  of either Andrew or of the character.”

Whatever they produce, the new series is being almost obsessively anticipated by its fans, some of whom are outside the London hotel where these interviews were taking place. “They don’t teach you how to deal with that,” says Cumberbatch, who undoubtedly owes his star status to Sherlock. Does he find the obsessive  nature of some Sherlock fans just a little, well, too much? “I had a friend who once squeezed her rabbit too much until it started to squeal and she thought it was kind of going, ‘I love you’, when it was really saying, of course, ‘You are the reason I’m dying’,” is his slightly alarming response. “But what I love about the show is that there are lots of people who weren’t outside the hotel today equally excited to see it and are just waiting for it as a quality piece of television.

“The problem, of course, is he [Sherlock] uses social media and it gives a platform for this fan fiction, which is really creative but it’s not really what we’re doing… [online fan fiction that imagines Watson and Holmes as gay lovers]. It’s part of the love people have for the show even if a few of them are quite fanatical about it.”

Freeman is also vexed by the constant monitoring of himself and his character, saying: “‘Can I have a picture?’ is the same as, in my day, ‘hello’. And people do look at you askance if you say, ‘no… not today.’ It’s like you’ve just taken food away from their children.” However, both actors fully understand the appeal of the show. “It’s because it really pays homage to the original,” says Cumberbatch. “And the fact that we’ve got three of the most talented writers in the country who happen to be fans.” Adds Freeman: “The visual aspect can never be underestimated. That’s been influential – text on screen now is a regular part of television. It wasn’t three years ago. You always had a cut away to a screen or a telephone.”

Cumberbatch has had a busy year since the last series, including playing Julian Assange in the art-house flop The Fifth Estate and Khan in the multiplex goldmine Star Trek Into Darkness. “They all come with their own complications,” he says of the roles. “One’s an incredible story of our time involving real people, in the other I was playing with all those toys with JJ Abrams at the helm, being dragged across the floor at nearly 50 km an hour and beefing up… I went up four suit sizes in the space of a month. I loved it.”

Was it hard morphing back into Sherlock? “Easier than the second season,” he says. “The writers are giving us such a wonderful time and when you hit the sweet spots it’s just a wonderful thing to be part of.” Freeman is more aware of living up to expectations. “Now is the difficult third series,” he says. “When we started it was like the world is your oyster. But now people expect certain stuff and want certain things. The danger would be if we were like, ‘this is all set in stone’, but what I know we’re finding in this series is that there is stuff to discover.”

‘Sherlock’ returns to BBC1 on New Year’s Day at 9pm

Observation and deduction: what we know about Series 3

Episode One The Empty Hearse

Based on “The Empty House”, Sherlock Holmes’s comeback short story after Conan Doyle killed him off 10 years earlier, the opening episode will reveal how Sherlock survived his fall from St Bart’s. Mark Gatiss has said that the London Underground will have a central role in the episode, while Sharon Rooney from My Mad Fat Diary will guest.

Episode Two The Sign of Three

Sherlock attends a wedding (as revealed in a Tweet by the producer Sue Vertue) in an episode which takes its title from the Conan Doyle story “The Sign of Four”, in which Dr Watson met his future wife Mary Morstan. It seems likely (but not certain) to be Watson’s wedding  to Morstan, with the added piquancy that Morstan is played by Freeman’s real-life partner, Amanda Abbington.

Episode Three His Last Vow

The word-play here refers to the Conan Doyle story “His Last Bow”, which was Sherlock Holmes’s last ever adventure (fear not, the TV show is returning for a fourth series). In this Steven Moffat-scripted tale, the detective turns spy in order to help out his country. The Danish actor Lars Mikkelsen, from Borgen and The Killing, will play blackmailer Charles Augustus Magnussen.