A Discovery of Witches has always had little bit of magic about it. Deborah Harkness' debut novel was a New York Times best seller on its release in 2011, with Warner Bros snapping up the movie rights shortly after publication.

"There was a kind of stampede when the book was first out," Harkness tells Digital Spy. "There was an 18-month option at Warner Bros and they hired a Pulitzer Prize-winning playwright to do the script... but it just wasn't possible to squash that book into 90 minutes.

"It wasn't for lack of talent, or lack of trying... it was just utterly defeating and so we paused and hit reset and had a moment to think and reflect."

In the aftermath, Harkness says: "I was pretty clear it should go to television."

ALAIN JOCARD/AFP/GettyImages

[Deborah Harkness]

Seven years later and a TV series inspired by the book has finally materialised. Filmed in Wales, but also on-location in Oxford, Venice and New York, Sky One's A Discovery of Witches stars Teresa Palmer (Hacksaw Ridge) and Matthew Goode (The Crown) in the lead roles of a witch and a vampire who find themselves inexorably drawn together.

The original book, and its two sequels Shadow of Night (2012) and The Book of Life (2014), follow Diana Bishop (Palmer) and Matthew Clairmont (Goode) explore Diana's family legacy, get caught up in supernatural battles and embark on a journey through time.

"It's not really my genre, the horror genre," says Goode. "I do remember seeing a clip of Nosferatu when I was very young and that scared the living shit out of me, so maybe that's lived with me!

"I'm not saying this is horror... this is more family fantasy. I have no idea how you peg this down, because there's a lot of strata to it."

That melding of genres – fantasy, horror, romance – made Harkness' source material "a very difficult book to adapt", but both Goode and Palmer say that having the author herself on-set, providing creative input, helped the process along.

"It's best not to have a writer involved if you can – especially having them on-set," says Goode. "It can get a bit... not with Deborah, I'm saying with other ones, because it's their material, it's their world, and so you end up having a lot of back and forth and tears. It can be very difficult, because it's not how they imagined it.

"Quite rightly, there's a lot of sensitivity around it. But actually Deb was a real help to us and a goldmine of information for myself and Theresa."

"She was our guiding light," Palmer agrees.

Sky One

Inevitably, the eight-part series has had to make some changes to the 700-page novel, with executive producer Lachlan MacKinnon describing the decision-making process of what to omit as "extraordinarily tough".

"There is such a huge fanbase and we want to be respectful to that audience, because they're so invested in it," he says. "There will be some changes... but we've worked so closely with Deb on it to make sure that she also feels that it will work for her audience, because she knows them so well."

Playing Sarah Bishop, the aunt / adoptive mother of Palmer's Diana Bishop, Alex Kingston reveals: "There's a very strong element within the Bishop household that is not present [in the TV series] and is very much present in the book.

"I know there was a lot of back-and-forth about whether this particular element should be added or not, but it was something that was going to be so costly, and also in a way would slightly detract from the drive of the narrative, that the decision was made not to include it."

There'll also be some "embellishments" to the source material, MacKinnon confirms, with the TV series expanding perspectives beyond just Diana's, who is the POV character in the novel. "There's no way we could do this [just] from Diana's point of view and make it work," says Harness. "We're trying to add to the value of the story, and expand on it."

Sky One

Despite the alterations made in transferring A Discovery of Witches from page to screen, Palmer insists that the small-screen version "feels pretty similar" to the novel. But Harkness herself is keen to emphasise that the plan is for the series to "reach people who maybe the books didn't reach, for whatever reason".

"I mean, we're talking about a couple of million people who bought those books," echoes Goode. "With television, you're opening yourself up to a completely different threshold. So we can't just make it about pleasing the people who know the books, because we want to make it accessible for people who haven't read them."

The good news for fans of the books, and for those new fans about to be converted by the series, is that a second season of A Discovery of Witches is already in the early stages of development.

Related: Here's how to watch Downton Abbey star's new show A Discovery of Witches

"We think it will be ongoing," says Palmer, while Harkness is optimistic that her latest novels – the fourth book in the universe, Time's Convert, is out on September 18 – could inspire future seasons.

"It's an expanding book universe that may or may not affect what they want to do with the characters on the screen," she explains. "The next book picks up about a year after the end of the third book, and so there would be a way that they could just keep rolling along if they wanted to."

For her part, Palmer is keen to explore the "boundless powers" of her character for years to come. "She's the most powerful witch in the world... she can do anything. So I'm really looking forward to seeing how that further develops."

A Discovery of Witches begins tonight at 9pm on Sky One and is also available to watch on NOW TV.

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