Benedict Cumberbatch is practically vibrating with manic joy.

Theyre having the time of their lives, teases showrunner Steven Moffat.

Sometimes thats portrayed as a kind of jealousy thing, which is a bit weird.

ALL CROPS: Sherlock Season 4 Trailer (screen grab) CR: BBC

Credit: BBC

Theyre all adrenaline junkies and thats bound to go wrong, he says.

You cant live your life like that and have it work out forever.

There will be consequences for that.

You know, its not a fairy tale.

There will be a darker side for rest of the [season].

Part of that darkness comes from the introduction of a new villain played by Toby Jones.

Moriarty features in very little of the Sherlock Holmes stories, Moffatt replies.

Hes not the villain very often.

So thats never bothered me too much.

You dont really need a master villain.

Sherlock doesnt need an arch-enemy, even though he has the most famous arch-enemy in fiction.

And we already know Moriarty.

When you introduce a new villain theres more mystery about them whats their point going to be?

What are they up to?

How is Sherlock is going to defeat them?

You cant have the Batman vs.

The Joker every week, you get depressed when the arch-enemy never seems to notice that they always lose.

It just becomes predictable.

Theres a whole host of less-famous ones in the stories that have never been touched.

Sherlocks first new episode in a year, The Six Thatchers, premieres Sunday night on PBS Masterpiece.