A Spectre calls: How His Dark Materials brought Philip Pullman’s terrifying monsters to life

“For the Spectres, , you read the book and you try and understand the basic rules of them which is, you know... fundamentally it’s that they’re not really described as humanoid at all, and kids can’t see them until they hit puberty,” VFX supervisor Russell Dodgson told RadioTimes.com.

“The trickiest thing with them was working out how they looked, and how you make something look different. Because we didn’t want to do something humanoid.”

Specifically, Dodgson revealed he was worried about making the Spectres look too similar to other similar fantasy creatures, most notably a certain gang of Azkaban guards from the Harry Potter series who are often described similarly.

“There’s also the danger of just making Dementors, which is something we want to avoid!” Dodgson said. “It is tricky.”

In the end, Dodgson and his team settled on imagining the Spectres as a kind of viscous, shapeless fog barrelling through the sky, creating a truly unsettling image – but even this was hard to achieve without the kind of references they’d been able to use for the daemons or armoured bears in earlier episodes.

“Working out the different ways in which they move [was tricky]” Dodgson told us. “And then when you do all that with [digital] effects, inherently it’s unpredictable.

“Because a lot of it is sort of algorithmic and procedural. You put in the values that you think, and you get a result and you think ‘this is really cool!’

“And then you get the same one from a different angle and it’s just utterly bananas, and it doesn’t make any sense. So yeah, it’s a bit of a ride.”

Possibly because of this arduous process, unlike the other supernatural creatures in His Dark Materials (which are puppeteered on set) the Spectres weren’t represented during filming at all, meaning the actors had less to go on when formulating their responses.

“There wasn't really anything on set, so we had to imagine it - because I don't think they had developed what the Spectres would look like while we were filming,” Amir Wilson, who plays Will, told RadioTimes.com.

“I think that came after. So no, there wasn't really much to look at. We don't have too much interaction with the Spectres, but there's a scene where Will gets to chase a Spectre with a knife, and kind of threaten a Spectre.

“I was actually pointing a knife at nothing, but I got to imagine what it would be doing, and they told me how it would move and how it'd look, so I imagined that in my head.”

In a way, though, this was less of a handicap than it sounds, as within the series the young Will and Lyra aren’t old enough to sees Spectres anyway. Call it all Method acting…

“I got to interact even less than Amir did, but I got a few scenes, because obviously the younger you are the less you see Spectres,” Dafne Keen, who plays Lyra, told us.

“So by the end of the season Lyra can sort of see shapes and things. But like Amir said they didn't even know what it was going to look like, they basically just told us it's going to be there and there, and we said 'what are we looking at?' and they said 'we don't even know yet.'”

“It was quite weird because we had to be really scared of something we didn't know,” she added. “And it was really fun investigating that.

“I was in a scene where Amir was chasing around a Spectre and it was really technical, really fun to film, really spooky. There were all these moments where Amir was seeing Spectres and I was having to be like, 'what are you doing?' A very creepy thing.”

Sounds like a total Spectre-cle.

Read more about the His Dark Materials cast, the His Dark Materials release schedule, the His Dark Materials books and the His Dark Materials age rating, plus find out where His Dark Materials is filmed, including the scenes set in Cittàgazze.

His Dark Materials continues on BBC One on Sundays at 8:10pm. While you’re waiting, visit our TV Guide to see what’s on tonight, or check out our guide to new TV shows 2020 to find out what's airing this autumn and beyond.