It has been made clear time and again that the upcoming fourth season of "Sherlock" is going to be the most intense in the series so far. Co-creator Steven Moffat has echoed that statement once more in a recent interview, while series star Benedict Cumberbatch previews what's in store.

While speaking to Digital Spy on the set of "Sherlock" season 4, Moffat teased that the fourth season is going to be unlike any other.

"Without giving anything away, this is about as tough a journey as he goes on. Sherlock and John both, actually. This is about as difficult as it gets for them. They kind of go to hell and back this time," he said.

Cumberbatch also gave a little hint about the gravity of the forthcoming season. "If what we've seen before is an unravelling, then it's a drop in the ocean compared to what's to come," the "Doctor Strange" star teased.

The new season doesn't only have a new adventure and a new villain, though, because there is also going to be a new cast member: John (Martin Freeman) and Mary's (Amanda Abbington) baby. It was revealed at the couple's wedding in the previous season that Mary was pregnant with John's child. The upcoming season is going to see the pair handle parenting, all while facing off against Toby Jones' Culverton Smith.

"There are a lot of new things going on, for example there's a baby! So parenting responsibilities have kicked in for the super sleuths! Child care is never easy, but it gets even more complicated when crime's involved," Cumberbatch told The Hollywood Reporter.

It has been previewed in the trailer that the fourth season will be more emotional than past ones. The clip even features Sherlock saying "I love you" to the camera, although it remains unknown to whom it is actually addressed. 

"Sherlock" season 4 will premiere on Jan. 1, New Year's Day.