“I mean the Louis Jordan 1977 BBC version, which is tremendous, is the most faithful version there is. But just about every other version takes its own route. And that's partly because a lot of what we think of as Dracula is actually based on the Dracula play rather than the Dracula novel.

“The tradition is you take your own way with it, and we've done that.”

“That's true, in that sense it's very traditional,” Gatiss agreed. “It is quite freeing, because people have maybe a general perception of it, which is useful - because they know all the big beats, I suppose. But they're not slavishly adhering to the book in that way.

“Honestly, in a very Sherlock way it's faithful and faithless at the same time. We absolutely love it, we love every aspect of it, like with Sherlock Holmes. And it is actually often quite faithful – and then also not.”

And happily, we have plenty of examples of both.

Episode three - The Dark Compass

As is to be expected, Dracula episode three is by far the furthest departure from the book, catapulting the Count into the 21st Century (which obviously never happened in Stoker’s text) and making major changes to the characters.

However, a few things are retained from the source material.

Lucy Westenra

As in the novel, there is a 22-year-old Lucy Westenra entertaining a number of suitors including Dr Jack Seward (here a junior doctor played by Matthew Beard, on the page an older physician specialising in psychological disorders) and Texan Quincey Morris (Phil Dunster), though the man she actually chooses in the book (Arthur Holmwood, Lord Godalming) is cut, bar a reference in episode one to a ‘Dr Holmwood’.

As in the book, Lucy also makes visits to Dracula in a graveyard and gradually succumbs to his charms before becoming a vampire – though on the page she did so while hypnotised, not as willingly as she does here, and the graveyard is in Whitby. The whole sequence of Lucy’s cremation and resurrection is a new invention for the screen, with the book version of Lucy becoming a “voluptuous” vampire who is eventually killed and laid to rest by her three suitors and Abraham Van Helsing.

One more key detail – the repeated use of “bloofer lady” as a synonym for beautiful lady – is lifted directly from Stoker’s text, where it is used by Lucy’s child victims when she first begins preying on them as a vampire. Here, it is an undead child who first says it to her before she starts to use it to describe herself.

Count Dracula

Almost all of Dracula’s storyline here is new material, bar the interactions with Lucy Westenra mentioned above. In the book there is no Jonathan Harker Institute, no attempt to capture and study him and Dracula ends up being killed in a chase back to Transylvania when Lucy’s suitors, Mina, Van Helsing and Jonathan team up to destroy him.

Still, there are a few nods to the book here and there.

“Ah, the Children of the Night – what music they make,” is a line Dracula says both onscreen and in the source material, though in the book he’s referring to howling wolves and speaking to Jonathan Harker in Castle Dracula, while in the TV adaptation he’s drawing Lucy’s attention to the undead trapped in coffins.

The central idea of the final scenes, where Sister Agatha (Dolly Wells) reveals Dracula’s great fear of and secret longing for death, arguably has some history in the book as well. Whenever a vampire is killed, it’s noted by characters that a look of relief and peace passes over their face, and at the conclusion of the book Count Dracula is no exception – clearly, sometimes it’s nicer to actually lay at rest.

Episode two - Blood Vessel

Even more than the first episode, Dracula episode two makes some huge changes to Stoker’s text – not least because in the original novel, Dracula’s bloody time on the Demeter is only a tiny portion of the story, told in retrospect through a discovered Captain’s log.

“We've taken some joy in expanding four pages of the book, which is what happens on the Demeter, the sailing ship that takes Dracula to England,” Moffat told us on the set of Dracula.

“It is literally four pages of the captain's log,” Gatiss added. “It's a great bit. But you don't hear much about it.”

• Dracula episode 2: 8 big questions we have after Blood Vessel

The Demeter

Apparently the sequence of Dracula gradually picking off the crew one by one was too delicious an image for Gatiss and Moffat to avoid, with the pair noting its similarities to a more modern monster story.

“It's such a great idea - essentially Alien on a sailing ship,” Gatiss said.

“We've just expanded it into a whole episode. And that has an entirely new cast of people who aren’t in the book, or might have been.”

Accordingly, nearly every crew member and passenger in the episode is a new invention in the series from Sacha Dhawan’s Dr Sharma to Catherine Schell’s Grand Duchess. In fact, only the Demeter’s captain – played by Sherlock’s Jonathan Aris – exists in both versions of the story, with a similarly heroic last stand in both (although the details are quite different).

In fact, the Demeter of the book doesn’t even have passengers, just crew, and the ship we see onscreen (built from scratch in Bray Studios) is probably quite different to the one Stoker had imagined.

“I think the ship that Stoker had in mind was probably more like a clipper,” Gatiss said.

“But we've made it into more of a passenger vessel-cum-cargo. It is slightly Hollywoodised, just because you have to, but it's not ridiculously big.”

“There would have been ships of this size,” Moffat explained. “But in terms of height of the rooms down there, they're all there to accommodate a 6 foot four Dracula.

“Because it would be a less good, and less sinister Dracula if he was banging his head all the time.”

Dracula

And the change to include passengers also benefits a certain Count Dracula, who in this retelling plays a far more active role in proceedings.

In the book, Dracula travels incognito within one of the earth-filled boxes (notably, the crew’s discussion about them containing “mould” is a detail lifted directly from the text) in the ship’s hold, never revealing himself except to feast on an unlucky crew member (none of whom, it’s worth noting, particularly resemble the crew we meet onscreen).

In this adaptation, Dracula instead travels as a passenger, and this change is referenced in a slightly tongue-in-cheek line Bang’s vampire says to Agatha (Dolly Wells).

“It’s four weeks to England,” he tells her. “What did you think I was going to do, lie around in a box?”

Accordingly, all of Dracula’s attacks, one-on-one conversations, his “investigation” into the murders and his eventual clash with the crew is all made up and extended, coming solely from the minds of Moffat and Gatiss.

“It's quite odd that Bram Stoker didn't run with it himself,” Gatiss said.

“Other versions have - in Nosferatu there's quite a bit, famously where he comes out, and it rather marvellously has the intertitle 'the ship of death has a new captain.'

“It seemed an irresistible thing to expand on really.”

Sister Agatha

Of course, the framing sequence of Dracula and Agatha’s chess game isn’t in the novel – in fact, neither Sister Agatha nor Abraham Van Helsing (who are combined into one character in this TV series) have any part to play in the journey of the Demeter, and certainly don’t end up battling Dracula on deck.

Really, when it comes to Agatha, it’s more rewarding to try and find anything that is taken from the book – and from this episode we’ll land on the fact that her hallucination of talking to Dracula in his wine cellar is a reference to a quote in Stoker’s text that a woman is the Count’s “bountiful wine-press.” Though that could be a coincidence.

The future

Appropriately, Agatha is also central for this episode’s biggest change of all – the scene we’re calling Drac to the future.

It’s no spoiler to note that in Stoker’s novel, the voyage of the Demeter doesn’t end with Dracula arriving in the present day. In fact, by contrast the novel sees him bound off the decks of the empty Demeter (after he killed all the crew) in the form of a dog, running into Whitby to begin his campaign of terror against Lucy Westenra.

In this new TV version he’s delayed somewhat by the sinking of the Demeter, which apparently strands him underwater for over a century then lands him in the clutches of the curiously-still-living Agatha.

Clearly, the next episode may have a few more changes for us to pick through…

Episode one - The Rules of the Beast

Jonathan Harker

Much of what befalls John Heffernan’s unhappy solicitor in this episode also happens in the book, from his arrival at Castle Dracula at the hands of a red-eyed coachman (rumoured to be the Count himself), his conversations with the Count and his attempt to seek out the mysterious women living in the castle.

A sequence where the Count forces Harker to write three letters providing an alibi for his upcoming death is taken almost directly from Stoker’s text, right down to the town – Bistritz – where the final letter is to be sent from. Another scene, where the elderly Count shatters Harker’s shaving mirror (and Harker cuts himself) also comes from the book, where the character uses a similar excuse that it is a “bauble of vanity” that he has no time for.

However, there are some major changes as well – some, fairly understandable, given that few adaptations have included then in the last century anyway.

“There are other parts of the book where you think 'Well actually we can let that slide.'” Gatiss said. “There's some curious things which are never done.

“At one point, because Dracula can climb up walls, he's climbing up the walls of his own castle like a lizard, and Harker notices that he's wearing Harker's clothes - as a sort of alibi!

“Like a Columbo episode, to give the impression that Harker is still alive, Count Dracula, who's an elderly, very tall man who looks nothing like Jonathan Harker walks around the town wearing his outfit,” added Moffat.

“We'll probably not do that bit!” laughed Gatiss.

Harker’s investigation, map-seeking and attempted rescue of Dracula’s brides never happens in Stoker’s book, where it’s the Count, not one of the female vampires, who Jonathan spies climbing the walls.

It’s also never made clear whether Jonathan is fed on in the novel during his stay at Castle Dracula, and he’s certainly not physically diminished and turned undead in the way he is in this new adaptation.

In fact in the book Harker escapes Dracula’s brides (who aren’t kept like animals, as in this version) shortly after the Count himself has begun his journey to London, in a manner never made entirely clear in the text. By leaning into this uncertainty of Harker’s escape, Moffat and Gatiss actually create a new mystery that’s resolved when the truth of Jonathan’s new vampirism asserts itself – ‘Just how did he get out of Castle Dracula anyway?’.

Book-Jonathan does find his way to the convent in Hungary and the care of Sister Agatha, and Mina does come to find him there – but unlike the TV version he leaves alive and married, and continues as a major character for the rest of the book. He doesn’t become a vampire, Mina and Agatha aren’t in danger in the convent, and generally speaking on the page it’s a much nicer time for ol’ Jonny Blue-eyes.

Count Dracula

Claes Bang’s take on the vampire also has a few new twists, though some of these are extrapolations of what happens in the book.

“There's stuff that's adhered to Dracula over the years, that become part of the central story of it, that were never in the original at all, as with Sherlock Holmes,” Moffat said.

“It's a surprise when you read the book, because Dracula's out in the day light all the time, it's not a problem for him. That's from Hollywood.”

Like in Stoker’s novel the Count begins the story as an old man – however, on the page he doesn’t become younger until he arrives in England, instead maintaining his appearance as an elderly Transylvanian man throughout Jonathan’s time in his castle.

One detail no adaptation has included is that the book Dracula has a moustache, something also dropped by Moffat and Gatiss after trying to include briefly for Bang’s older-looking self. As a younger man, he looks and dresses very much like a traditional vampire – though, as Moffat and Gatiss pointed out, that’s not really from the books.

“Certain things that everyone likes, you keep,” Moffat said.

“No cape in the book,” added Gatiss. “The first movie to do fangs was a Turkish one called Dracula Istanbul, a very strange film. Literally no fangs in Dracula before that. Nosferatu's the first film to have him destroyed by sunlight. It comes from the movies, it comes from all different sources.

“That is actually 120 years of accrued stuff from the plays, from the first films, the adaptations. Dinner jacket version, Bela Lugosi version - lots of different things have become what we now think of as Dracula. But you have to honour them.”

As noted, in the book it’s not really suggested that he feeds on Jonathan during his time in Castle Dracula, and he certainly doesn’t absorb the young solicitor’s youth, accent and memories as Bang’s version does on-screen. In the book, Dracula also fails to explain his identity as a vampire, the nature of his brides or his relationship with the sun as this TV version does either, and generally speaking the characters have a more distant relationship.

The whole final sequence of Dracula’s attack on the convent is a new addition for the TV series (in the book he never visits there), though some of the weaknesses and abilities he shows there – a dislike of crosses, controlling wolves, shapeshifting into a dog, climbing walls – are taken from the source material at different points.

With that said, though, the way Dracula becomes a dog, hiding within its skin in a trick he also uses to disguise himself as Harker later on, is a new invention for this adaptation, and significantly more gory than you might expect.

Also, in a general sense Bang’s playful, pun-happy version of Dracula is a pretty far cry from the austere, grand Count seen in Stoker’s text. According to Moffat and Gatiss, this came from a need to make the Count the central figure of the story, rather than just a monster or villain to be vanquished, which presented one of the biggest challenges of the adaptation.

“We couldn't work out how he spoke at all – at all,” Moffat said. “We kept avoiding him. And then when we figured out a way for him to speak that made sense, he became really great fun to write.”

“Obviously he has these fantastic pronouncements - the ‘children of the night,’ the ‘like sheep in a butcher's shop’, or ‘my revenge is spread over centuries and time is on my side,'" Gatiss said.

“But he can't just say that sort of stuff all the time!”

Van Helsing

Perhaps the biggest change of all for this episode comes with Dolly Wells’ character Sister Agatha, an atheist nun who is revealed to be this adaptation’s version of Van Helsing, a vampire hunter and Dracula’s nemesis.

“In the book, she's a nun who looks after Jonathan Harker when he flees Castle Dracula and comes to stay in the convent,” Moffat said.

“In the book she just writes to Mina and says 'Your husband's turned up and he's babbling about this evil Count.' So we've expanded the part quite a lot.”

Accordingly, this new version of the tiny character who interviews Harker, taunts Dracula and draws Mina (Morfydd Clark) into her schemes is almost entirely a creation of Moffat and Gatiss, albeit combined with Stoker’s character Abraham Van Helsing.

“I didn't know she was Van Helsing. I knew I really wanted to be her,” Wells said of the role.

“I knew she was a crazy cool woman, because the audition scene was the convent, the two scenes. And I thought this is the coolest woman I've ever seen on the page - it was so exciting.”

In the novel, Van Helsing is an old Dutch scientist and doctor who uses his knowledge of the occult to help some of his friends fight off and eventually kill Count Dracula. In popular culture he’s since evolved into the prototypical vampire slayer, and has been played by numerous actors including Peter Cushing and Anthony Hopkins (as well as Hugh Jackman that one time we don’t discuss).

Now, by combining him with Agatha, this adaptation brings the character into the action much earlier and in a different location (Hungary, rather than England), setting Dracula up against this threat sooner.

“We came up with the idea of atheist nun - we just called her 'atheist nun', the nun who didn't really believe in anything and just made lots of jokes,” Moffat said of their new combined character.

“And that character immediately leapt to the top of the script. You just thought ‘Oh my God, that's great.’ And very very quickly we just thought well... that's Van Helsing, isn't it? We don't need anyone else. We just need the nun.”

At the end of the first episode, Agatha and Mina’s fate at the hands (and fangs) of Dracula looks far from rosy (in another change from the book, where Mina only crosses paths with the Count back in England) – but we feel like this new Van Helsing, at least, might live to create more page-to-screen updates in future episodes.

Dracula continues on BBC One on Thursday 2nd January and Friday 3rd January at 9pm