How His Dark Materials created the series’ Subtle Knife portals

But how to bring that effect, so perfectly described in Philip Pullman’s original text, to life? Well, for the team at Bad Wolf Studios in South Wales it was simple – a bit of imagination and a lot of fishing wire.

“My favourite scenes were all the stuff with opening the knife-worlds, the knife stuff,” Amir Wilson, who plays knife-bearer Will said. “Opening portals and stuff - that's really cool.

“It is kind of a secret... but they make a little window out of wire. They make a fence out of wire and they can adjust the size, into how big we want.”

Within the world of the show, Will’s use of the knife is visualised as him selecting different “threads” from the air, piercing them with the knife then slicing down them to open a portal to another world. On set, the threads and portals (or windows) were real, created with wire by the VFX team to allow the actors to more easily interact with them.

“All we did for that was we had a piece of fishing wire, which we would attach to the ceiling and the ground,” VFX Supervisor Russell Dodgson told RadioTimes.com. “It’s all you really need when someone cuts a window, they need something to look at, something to put a knife against that gives them some physical resistance with their hand. And something to run it down. That’s all you need.

“If you use fishing wire, we can clean it up really easily, they can interact with it, they can see where it is. We always ended up making little custom rigs that worked for that particular scene, and try and keep it as relaxed as possible.”

And if the finished effect looks a little familiar, that's no accident...

"If you look at the window effect and you look at the title sequence, they’re connected," Dodgson said. "And when we use our angels [later in the series] we start referencing how the windows work, because they have this kind of interdimensional quality that we’re trying to represent."

Dodgson, whose team at Framestore also create the animal daemons seen in the series, noted that the approach was similar when creating the fishing wire/knife effect, combining CG work with physical on-set props (in the case of the daemons, puppets were used) to help the actors realise Pullman’s imaginary world.

“It’s kind of the same with the daemons – we offer up puppets because they’re lo-fi, but we get a lot from them,” he said.

“We always try and keep everything really simple, to be honest. Because the rate at which we shoot in television…we try and keep things nice and simple, both for the directors and the actors.

“A big part of visual effects is just not being in the way. Making clear what you need up front, making clear what you need after, and trying to make yourself as helpful but unobtrusive while shooting.”

Really, the whole thing brings new meaning to the phrase “down to the wire”…

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.