Steven Moffatt thinks new 'Sherlock' villain Toby Jones is "unnerving".
The showrunner thinks the 50-year-old actor's Culverton Smith is the "darkest villain [they have] ever written" for the show and thinks fans will enjoy his appearance in the upcoming festive episodes of the popular drama, which stars Benedict Cumberbatch as the titular detective.
He said: "He's very unnerving. I hope Toby won't mind me saying, but he's a little man, yet he's weirdly a physical threat."
"He exudes an enormous amount of menace."
And co-creator Mark Gatiss - who also plays Sherlock's brother Mycroft - thinks Culverton's humour just adds to the menace of the character.
He told Empire magazine: "He's also extremely funny. The switch between the two can be terrifying."
Moffat has ruled out any hope for fans that Sherlock's nemesis Moriarty could make a surprise comeback.
He insisted: "Moriarty is dead. Shot himself through the head. He is dead."
Also on the festive episodes, there is joy for John Watson (Martin Freeman) and his wife Mary (Amanda Abbingdon) when they welcome their first baby into the world - much to his best friend's upset.
Martin said: "It inevitably changes the relationship between John and Sherlock.
"For John, as you can imagine, there can be nobody more important than his wife and child."
According to Benedict, Sherlock greets the news of the new arrival with "mild indifference".
The actor also promised the new series will "deliver an enormous number of pay-offs" for fans, though they won't all be happy.
Moffat added: "This is about as tough a journey as John and Sherlock go on. We're taking them to hell and back this time."