This image may only be used for publicity purposes in connection with the broadcast of the programme as licensed by BBC Worldwide Ltd & must carry the shown copyright legend. Image Credit: Hartswood Films

Sherlock fans, mark your calendars: The show is returning to the BBC with series four in January 2017, if all goes according to plan.

“We start shooting in April, finish shooting at the end of July. There’s three of them,” said producer Sue Vertue on Tuesday, speaking at the BBC Worldwide Showcase in Liverpool.

“They feature Sherlock Holmes and Dr Watson, extensively,” chipped in writer and producer Mark Gatiss. “That’s about it.”

Gatiss, who also plays Sherlock’s brother Mycroft, said that we can expect more interactions between the two.

“It’s more hinting at stuff. We decided early on that it’s much nicer to open little windows into the past, than it is to just say everything. People have their own theories, and we know bits and pieces about what their strange upbringing was like,” he said.

Gatiss and Vertue, joined by writer-producer Steven Moffat at the Liverpool event, offered us something to tide us over in the form of the Abominable Bride, a Christmas special that reverse engineered their re-imagination of Sherlock and took us back to Victorian times.

But before that was series three, characterised by Sherlock’s return from the dead, the introduction of Mrs Watson, and the resulting tug-of-war between the two over Dr Watson’s affections.

According to Gatiss, original Sherlock Holmes author Arthur Conan Doyle makes a “rare mistake” in the books when he marries off Watson too early.

“He has to set a lot of the stories in this nebulous period where they’re still sharing a flat just before the wedding, lots of stuff happens. It goes on for years,” he said.

“We thought, approaching the third series, it was the perfect time to introduce Mrs Watson. Sherlock’s dead. Two years have gone by. Dr Watson is moving on from this terrible trauma. Wonderful person enters his life, and it’s just the worst possible time for Sherlock to show up ... It’s a threesome, really,” Gatiss said, adding, “Not in that way.”

When Sherlock first aired in 2010, Benedict Cumberbatch was as fresh-faced as they came. What did he bring to the role?

“He brought his own trousers,” quipped Gatiss.

“It’s Sherlock in his early days, isn’t it, and [Benedict] does bring this kind of puppy enthusiasm and energy,” said Vurtue.

“But still, with a kind of Byronic look. It was a combination,” added Gatiss.

“That being said, an awful lot of that stuff you’re liking was invented by Arthur Conan Doyle over a hundred years ago,” said Moffatt. “A lot of those things — all the jumping on the sofa — that’s all lifted from the stories. We can’t claim to have created that, and Benedict can’t either. It was there.”

In the early days, Gatiss and Cumberbatch exchanged a “few emails that said, ‘[Sherlock’s] not very nice, is he?’ And well, that’s the point, and that’s the reason people have always liked him. He says the thing that nobody else will say in a room. He’s just very naive in that way, he just says it, because he has no filter. Dr Watson eventually gives him a little bit of a filter.”

As for how long it takes Gatiss or Mark to write a single episode, the answer is however long it takes from the moment they have to start writing to the moment, several deadlines later, that they have to hand it in.

“I was just moaning to Mark that given how slowly I write, I don’t know how it is arithmetically possible that I’ve written so much,” said Moffat. “I look at what I’ve done in a day, and I think, ‘It’s shorter and less good than when I started.’ But I’ve written quite a bit of telly over the last few years. I don’t know how. Something’s happening. Elves are coming in at night.”