Two years after a Christmas Special last year that saw the series travel back in time to the Victorian era to sojourn with zombie brides, the latest offering based on Arthur Conan Doyle's murder mysteries will kick off with a guest star in the form of the offspring of John Watson (Freeman) and his wife Mary (Freeman's real-life partner, Amanda Abbington).

"They have a baby girl. You know it," confirms actor Rupert Graves, who plays policeman Lestrade on the series.

"Because someone took a photo of it and outed it on the Internet..." he adds slowly.

He glares, mock-daggers, at a gaggle of international press in a London hotel conference room, making light drama of (deliberately) leaked paparazzi photos during an earlier film shoot in Borough Market.

But as everyone in the room expects - the baby in Sherlock does throw in a number of awkward plot moments. Not least because the vaguely sociopathic title TV character already has issues relating to general humankind on an everyday level.

In a separate interview, Sherlock's actor Cumberbatch - himself recently a new father - alludes: "I certainly handle babies on set very differently to how I handle them in real life. But let's not go into that..."

To which - "What baby? Whose baby?" is Freeman's response.

"There were so many babies on the set and they were all interchangeable, it all became quite confusing. Male, female. Boy, girl. I don't know. I never knew," he adds.

Freeman's partner Abbington - who already has two of her own with him - throws in her two cents' worth as the TV mother of the child in question: "At least you could give them all back after you were done filming. Not like in real life."

(Read also: Doctor Strange is not Sherlock with magic, says Benedict Cumberbatch)

But "it does get us together a little bit - in the first episode anyway", says actress Louise Brealey, who plays Molly the pathologist.

And do not worry, she reassures all - "they don't overdo the nappy humor".

For all the lighthearted banter about milk bottles and projectile pi**ing, however, Sherlock's fourth outing is allegedly darker - in fact, "very, very dark", as Graves says.

"I got goosebumps, proper goosebumps, reading the script. And it doesn't really happen very often," he adds.

TV teasers hint at the usual conundrum of whether the series' arch- villain Moriarty is back for vengeance. But beyond that, an ominous edge reeking of creepy horror - as opposed to gritty crime - glosses each scene.

On the trailer, a voice whispers: "Something's coming. The roads we walk have demons beneath."

Cumberbatch points out that "it wouldn't be so satisfying if we were churning out the same things we started with".

"We keep on trying to surprise ourselves and our audiences. The level of invention, the imagination, the darkness and joy... there's lots of serious stuff, but which they still managed to turn into something funny. There's something still very human about this."

In preparation for the fourth incarnation of this role, the actor says he had to "keep learning to do new things".

Citing the lack of a photographic memory, he says he struggled on set to simply become a disgustingly clever person, albeit with one or two payoffs.

"I'm not a natural learner, I have to try quite hard. I feel my brain is a little more subtle and capable when I'm playing him.

"Whether it's piano music or learning more French. I do feel more alert, I'm keyed up," he explains.

There are also other kinds of kickbacks. "My mother says I'm much more curt with her when I'm filming Sherlock."

And yet that is perhaps why TV audiences watch and look forward to the title character's acerbic and arrogant one-liners, mostly thrown at the world-weary Watson.

(Read also: Meet Charlotte Holmes, Sherlock’s granddaughter)

Indeed, for those who enjoy the show for the wonderfully strained love-hate play-off between Sherlock and Watson, rest easy that this element will very much remain in play in the new season.

Cumberbatch says: "The jarring nature of their friendship is endless fuel for great drama. That's probably more heightened than the originals."

He adds: "That's the thing - they're an odd fit. There's one crossover and that's danger. The lure of danger."

And it is danger that exposes - ultimately - Sherlock's own fallibility and humanity. Yes, the man does make mistakes.

Cumberbatch argues: "It'd be incredibly dull if he was permanently an incredible hero and always right and the episodes were always just about the unraveling of other people's stupidity to realize that he was right."

Sherlock will return in The Six Thatchers on January 1, 2017. #Sherlock pic.twitter.com/s6RJdaOZdI

— Sherlock (@Sherlock221B) October 26, 2016

Popular

Volcano erupts in North Sulawesi, hundreds evacuated

Dubai roads, airport reel from floods after record rains

Ramadan boosts consumer confidence, retail sales