The cast and crew on HBO’s His Dark Materials knew it was going to end at some point. The fantasy series is based on a trilogy of books by Philip Pullman, with each season roughly corresponding to a particular novel. After a slight hiatus due in part to the pandemic — season 2 wrapped up in 2020 — the third season, which follows the events of the final novel The Amber Spyglass, premieres this evening on HBO. And for Amir Wilson, who plays Will on the show, heading into work on the finale came with mixed emotions. “I had come to terms with that,” he says. “When this ended it was going to be the end. But it’s sad to say goodbye.”

Season 3 is both the darkest and most complex in the series. For the uninitiated, His Dark Materials takes place in a sort of fantasy multiverse, where the existence of parallel worlds and a mysterious substance called Dust are only just being proven and understood. At this point in the story, characters are flung across various worlds. Will and Lyra (Dafne Keen) are venturing to the land of the dead to save a friend while Lyra’s father, Lord Asriel (James McAvoy), is amassing an army from multiple realms in order to start a war with God. (Seriously.) Meanwhile, the physicist Dr. Malone (Simone Kirby) is in a strange world trying to figure out what Dust really is and Mrs. Coulter (Ruth Wilson) is just generally causing trouble as she always does.

It’s a lot to keep track of, and things can be slightly harder to follow in the show versus the books. And it’s not just a problem for viewers. “It gets more and more challenging as the story goes on,” Amir Wilson explains. “I feel like I’m never fully up to date with all that’s going on. There’s always a little bit of confusion, because there are so many characters and so many worlds, you kind of don’t know who is when and where. But you somehow get the gist of it.” (Executive producer Jane Tranter notes that the script team, led by Becca Kinder, did an excellent job of “keeping track of everything both on the floor for everybody, and behind the scenes.”)

Ruth Wilson dealt with this by trying her best to stay in the moment. “Sometimes it’s just best to play the scene as is,” she explains. “On this, there were lots of scenes being worked on on the set as we were doing it, which is sometimes really frustrating, but sometimes it also feels very creative. Often, I think, you have to just play the scene, and be present in the moment. If you play it right, then it should stick together with the next scene.”

Outside of its complexity, the season is also notable for its sheer scale. It’s literally about a multiversal war on heaven. This means that, for viewers, there’s a lot more to ooh and ahh over, from mythical creatures to a new magical device in the Amber Spyglass. In terms of locations, the land of the dead starts out as a boring bureaucracy before becoming something much more terrifying, while Dr. Malone spends her time living with strange creatures called the mulefa, who roll around on seed-like wheels through a beautiful world. In short, it gets a lot weirder.

But for Tranter, there was one element in particular that had her worried going into this season. “I was anxious about the angels for sure, and how we were going to normalize the angels,” she explains. “But I think we’d learned by the time we got to season 3 that we worked very well as a creative team, and we could work our way through whatever kind of challenging situation that was going to come about.”

The other change, of course, is that everyone is older now. This is particularly true of Lyra and Will, whose actors are still teenagers that have been working on the series for four years now. “In season 2 he’s almost a child, and in season 3 it was important for me to understand and get across that this guy is older now, he’s a young man,” says Amir Wilson. “And bringing a newer, more mature version of Will to the screen was important for me early on. That was a driving factor for me throughout the series. I had grown up physically and mentally. I think Will in season 2 kind of went with the flow of things, when he maybe shouldn’t have, and this Will is more calculated and controlled in what he does.”

For Keen, her relationship with Lyra has changed over the course of the show and turned into something rather complicated. “I love her, I think she’s an incredible character,” Keen says. “But sometimes I’m like, ‘You are so irritating.’ It’s kind of like there’s a person that I’ve lived with for a long time and she irritates me. Ultimately I love her, but she does make really stupid decisions that just put me through it.”

This sort of changing relationship between actor and character was also true of McAvoy, who is a big fan of the original novels.

“I thought I knew who he was,” he says of Asriel. “He has a very strong character arc and outline as he is. But I kind of examined a few parts of him and aspects that maybe are hinted at in the books, but not really expanded upon; his jealousy of his daughter’s position as the real revolutionary force and icon, and his frustration and covetous relationship with Mrs. Coulter. And the frustration that he can’t have her and she won’t do what the fuck he wants her to do. It’s really interesting to explore the depths of that. And his mania — his megalomania in the book magically disappears when he needs to become a good guy and do the right thing. And we stuck with the megalomania a little bit more here in the show, and the mad scientist aspect.”

For Ruth Wilson, that kind of character — one with a depth and nuance that isn’t always readily apparent — is the ideal kind to play. “They’re the best characters, where you’re constantly searching in the process, constantly digging and finding new things, those hallelujah moments,” she says.

The third and final season of His Dark Materials also has the added burden of having to recreate a memorable and heartbreaking finale. I won’t spoil anything specific, but as those who read the books already know, Lyra and Will both have to make some tough decisions. People die, others are separated, and there are a whole lot of tears. (I’ve watched the entire season, and even though I knew what was coming, I have to admit, it was still a tearjerker.)

“You do feel the pressure,” Keen says. “There are so many people who love this: am I going to ruin it for them? But at the end of the day you have to kind of forget that and get on with the job, do what you need to do.” Ruth Wilson adds that, while “there are big expectations for how those scenes unravel,” she believes that “the passion and the care and the desire was all there from our side, and so I hope we’re giving the audience what they’re looking for — those who have read the books and those who haven’t.”

And now that their work on the show is done, the cast all have things they’ll miss. For Amir Wilson, that’s weekly games of football with the staff, while Ruth Wilson will miss her monkey companion, brought to life by the puppeteer Brian Fisher. McAvoy, meanwhile, is sad he won’t get to nerd out about the books so much now. “As a fan, getting to go cosplay as Asriel for a few years was fantastic,” he says.

His Dark Materials season 3 premieres on HBO and HBO Max on December 5th, with two episodes debuting each week through December.