Sherlock BBC

The writing duo found themselves hatching their plans for the fourth series on a train journey.

“We always seem to work best on trains,” joked Gatiss.

“It was a very strange day… and we mapped out the whole thing in one exciting journey.”

“Accidentally,” added Moffat. “We hadn’t meant to at all. It’s some of the best stuff we’ve ever come up with.”

Characteristically mysterious, Moffat and Gatiss kept their lips firmly shut when it came to any details about the fourth series of Sherlock, as did series producer Sue Vertue, who had joined them and Sherlock director Colm McCarthy for a question and answer session at Cardiff’s Chapter Arts Centre.

Excited fans had gathered there for a screening of the second episode. Also attending was none other next Doctor Who actor Peter Capaldi.

Gatiss and Moffat did reveal that next week’s episode, the final of this series, would officially introduce actor Lars Mikkelson’s big baddie, who is set to shake things up after the heartwarming events of last night’s episode.

“Don’t ever trust us,” said Moffat. “What would you think if we said after this everything would be the same?

“That would be boring, wouldn’t it?”

Last night’s episode saw Sherlock facing his biggest challenge to date - writing a best man’s speech for his crime solving partner’s wedding day.

There was all the usual intrigue and mystery Sherlock fans have come to expect, but juxtaposed with an insight into an unusual side to the sociopathic detective.

A sequence in which Sherlock and Dr Watson got well and truly drunk had the audience in stitches, while more heartwarming moments during the 85-minute episode resulted in audible gasps and “ahhhs” from the dedicated fanbase.

It wouldn’t be Sherlock without a mystery to solve, as Sherlock finds himself attempting to solve a murder before it even occurs.

During the Q and A, Moffat responded to criticism that last week’s episode, The Empty Hearse, was alienating the series’ “casual viewers”.

He said: “There are no casual viewers - it’s incredibly insulting to the audience to say that they’re not paying attention to the details.

“I think if you assume the audience is intelligent and paying attention, then you end up with a very successful drama.”

Last week 9.2m people tuned in to find out exactly how Sherlock survived his fall from the roof of London’s Bartholomew hospital marking another hit for BBC Wales’ drama department

Faith Penhale, head of drama at BBC Wales, reflected on the show’s success as she kicked off tonight’s screening. She said: “This Christmas has been an amazing one for BBC Wales drama, which has produced some of the highest rated TV dramas.

“These dramas, produced here in South Wales, have captured the imagination of the nation - not just here in the UK, but internationally too.”