Philip Pullman isn’t a planner. In September, the author of the His Dark Materials fantasy trilogy told The New Yorker that he couldn’t write a novel without making it up as he goes along. “I’m writing into darkness, as it were, not knowing where the story is going or what the characters are going to discover,” Pullman said. “It’s more exciting like that. I would just be too bored—terminally bored—if I knew everything in advance.”

Jack Thorne, the screenwriter of the new HBO adaptation of His Dark Materials, did know how the series’ story ended before he began the process of presenting it onscreen. That forced him to make difficult decisions about whether to stick closely to the structure of the trilogy or to tweak the portrayal of early events with later developments in mind. Although the opening episodes of the TV adaptation are in some ways more faithful to the books than the 2007 film adaptation, The Golden Compass, they do differ from the trilogy’s narrative timeline, mostly in service of exploring core concepts and characters that aren’t introduced in the first book, Northern Lights (known as The Golden Compass in North America and some other regions). The most momentous of those alterations aired in Monday’s Episode 2, when Mrs. Coulter’s accomplice, Lord Boreal, crossed over into “our” reality, the world of series coprotagonist Will Parry. Pullman depicted that transit in the second book, The Subtle Knife, in which Will makes his first appearance.

Thorne is no stranger to collaborating with literary legends on fantasy stories that fans hold dear: He wrote the script and cowrote (with J.K. Rowling) the story for Harry Potter and the Cursed Child. He’s also a frequent adapter of young-adult-oriented literature, having worked on the forthcoming films The Secret Garden and Enola Holmes. We talked to him about the appeal of Pullman’s trilogy, how he approached Pullman for input and approval, the challenge of fleshing out Lyra’s world without revealing too much, why we see Will’s world so soon, why he chose to emphasize or accelerate certain aspects of the books, and the importance of fantasy.

How did you get involved with His Dark Materials and how did you start to sketch out how you were going to adapt it and what modifications you might want to make?

So [executive producer] Jane Tranter is great, and we were emailing about something else entirely, and I said, “Great that you’re doing His Dark Materials.” I read their press release, and on that press release, it had this phrase which really stuck with me, which is that we’re going to “sound every note.” And that really appealed to me, and I said that they were amazing books. I really wasn’t fishing. Sometimes you do go fishing. I wasn’t fishing at all. But she emailed me back and said, “Would you want to talk about it?” And I thought about it and thought, no, not really. I was in the middle of doing Harry Potter. It was six months before we opened Harry Potter and the Cursed Child. I was knackered, and my wife was quite heavily pregnant, and I didn’t really want to go into another world like that. But the worlds were too enticing, too brilliant, and I couldn’t resist it.

And so I said, “Yes, please.” And she said, “How about next Tuesday?” And I think it was Thursday, and for the next however long it was, I locked myself in a room and read all three books again. And I’ve still got those books, the tattered remains of what they were. I started to, on the front of the book, just break it down into what I thought were segments that could work, particularly for Series 1. There are some changes to that, but actually that initial instinct is still a sort of shape of how we broke down how Series 1 would work.

What stage of life were you in when you first came across the books, and what did you think of them at the time?

I was in my early 20s, a bit depressed, living in my brother’s spare room, trying to be a writer. Fantasy has always been incredibly important to me. My son is called Elliott because of E.T. I’ve gotten “Be good” tattooed on my wrist. These books I just thought were beyond sensational. They transported me somewhere, and I absolutely devoured them. And then I bought them for my mum, and then devoured them again while she was reading them so that we could talk about it. I think they’re perfect, and I really hope we’ve done them justice.

It’s a beloved series, but in some ways it’s unusually structured. It’s not, “We’re setting off to cast the Ring into Mount Doom,” or, “We’re trying to defeat Voldemort,” where almost from the start you know what the goals and antagonists are. Central characters come and go, and some things are not clear from the beginning that you make a little more clear at the beginning of the show. What were some of the challenges of telling that story on TV, and what were some of the things you did want to rearrange?

The interesting thing is that Jane and I at the beginning did our first draft. The first draft I produced, that we worked on together, was incredibly loyal to what Philip had done. And then we had some meetings in L.A., and there were lots of questions asked of us. What’s this, what’s the Magisterium, what’s Dust, all this other kind of stuff. All the questions that Philip provokes. We went away and we worked on a draft which explained everything. And we worked for a long time on those. So we’d done about nine drafts before we went to L.A. Then we probably did another 15 drafts after L.A., which were just sort of trying to answer all those questions.

And then we sat down together and realized, this isn’t working. And the reason why it’s not working is because the hanging chad is essential to how Philip tells the story. That sense of perpetual—the ideas are out of grasp quite a lot. And people don’t know the answer to these things. They don’t know the answer to what Dust is. And so answering it or trying to answer it led us down some really wrong ways. But through going through that process, we’d also discovered some stuff that was incredibly useful.

The changes that were important to me to make were not ones we’ve mentioned. They were ones just trying to tease out the character a bit. So particularly Mrs. Coulter, just spending a little more time with her, understanding her a little better, and being given access to her emotions a little. For me in Episode 2, one of the most crucial scenes, and it’s what TV and film and the visual medium do better than books—it’s one of the few things TV and film do better than books—is just that moment when they have the bath together, Mrs. Coulter washing [Lyra’s] hair. And then you go with Lyra to her room, and then you cut back to Mrs. Coulter just sitting on the edge of the bath. And it’s all in the performance of the actor and it’s all in the truth of what they’re doing with their eyes. But it tells a thousand words. And so doing those kinds of things, that was what really mattered to me. That’s what I think gives this show the heft that it needs.

Things that didn’t show up until the second book make an entrance very early on in the show. At what point did you decide to show the crossing between worlds in Episode 2?

That was something that was really key to everything: Will, and Will’s world, trying to access Will. Will obviously operates along a parallel [path] to Lyra. The story you discover at the beginning of Subtle Knife, a lot of that story has happened during the process that Lyra’s going through in her story, in Northern Lights or Golden Compass, depending on where you live.

And I was fascinated by Will. These two are our heroes, right? They’re the ones that are going to take us through the whole book, the whole saga. And Lyra is given this opportunity, through Northern Lights, of really seeing how she’s made, how she’s made into that creature. By the time we meet her in Subtle Knife, she’s not fully formed by any means, but you can see what’s behind her style.

With Will, the opening of Subtle Knife is brilliant, and going into his world is brilliant. But I wanted to spend time with him. I wanted to understand how this fragile, brilliant boy is made in the same way that I wanted to understand how that fragile, brilliant Lyra was made. And so that meant accessing Will’s world a little. And that felt like an opportunity for us. And Jane and I took this idea to Philip, and Philip said to Jane that he would have—I don’t want to misquote him. He said that that is something that interested him, too, that if he’d known how the whole story panned out, he may have made different choices at the beginning.

He wasn’t against it. He thought that that was a really interesting idea, and that he didn’t know where the story was when he wrote the first book. And stories are a surprise, and that’s the brilliant way that Philip writes. He doesn’t have the whole thing in his head at the beginning. He discovers things as it goes along, beautifully, and I think the idea of bringing Will into the first series was interesting to him.

Could you tell me more about your interactions with Philip? Did you meet with him personally to discuss things? He’s working on The Book of Dust trilogy, so all of this must be very present in his mind. But how amenable was he to changes, and how much input did he have?

He had a lot of input. He read everything and commented on everything. He is a brilliant mind. And that thing of sitting down with someone who I think has written some of the finest novels of all time was an absolute delight. And the thing that we were trying to do all the time, and it was something that I’d done with J.K. Rowling a little bit when doing Harry Potter, was basically just find out the secrets. And the way you do that is not by going, “What are the secrets?” It’s by just trying to put together the clues that have been left. So with Philip, I remember a really thrilling meeting where it was trying to chart Asriel’s journey through Series 1. Asriel obviously appears at the beginning and the end of the book, but there’s a lot of reported stuff about Asriel through it.

And so I isolated all the times that Asriel had been talked about, and put all that in a document and then sort of drew a timeline on the basis of that, and then took that to Philip and said, is this timeline consistent with what you think? And of course it wasn’t consistent with what he thought, and that actually what he thought told us an awful lot more about this world than we knew, because there’s always hidden stories there. There’s always things that have lived inside his brain that haven’t come out on the page. And finding out those secrets was absolutely wonderful.

How did you handle the challenge of “showing, not telling” the rules of daemons and how they work in this world? These characters all understand their world, so they wouldn’t be sitting there explaining how daemons work, but you have to convey that to the viewer somehow. And you also have to keep in mind that there may be budgetary constraints, or it may be distracting to have CGI creatures constantly on the screen.

You can’t afford to have daemons on the screen constantly, no. There’s always the line where as a writer, with things that are laying down rules or things that can be characterized as exposition, always walking that line between telling people too much or telling people not enough. And so it was a delicate balance. We start the whole of the show with Lyra and Roger talking about what their daemon might fix as, so we’ve already got the idea of when the daemon might fix. And then we have the daemon-fixing ceremony for the Gyptians, where Tony Costa’s daemon is fixed, and so they’re celebrating the fact that his daemon is fixed. So we’re not overexplaining, but hopefully we lay enough ground that audiences will understand and go with us.

The Magisterium is sort of a remote, shadowy presence in the first book, but it’s a little more front-and-center as an adversary in the show. And it doesn’t seem as if you shied away from any of the parallels with real-life religion, which there’s been backlash about before. How did you decide how to handle that?

We just followed the books. We did bring them more into the light than perhaps Philip did in the opening books. That was partly to create an antagonist that audiences could understand, but also because Father MacPhail is another character that fascinates me. And like [with] Will, I wanted to tell his full story. By the end of [The Amber Spyglass], he is making some very interesting choices, some incredibly horrific choices. And I wanted to understand how that person came about. And so by starting the story slightly earlier and just [sticking] him into Northern Lights, Golden Compass, and Subtle Knife a little bit, too, I could really get under the nails of him a little and try and understand what he’s about.

There was an ongoing conversation when Game of Thrones was on about how much it should feature the book series’ fantasy side, which the showrunners resisted. With His Dark Materials, was there any hesitation about losing people who might not want to watch a fantasy story? What do you see as the point of fantasy, or the value of fantasy to viewers and readers?

I think the point of fantasy is to illuminate huge questions. So, what is truth? What is our essence? What is consciousness? These are huge questions that I think fascinate us all. And we live in an age when fantasy isn’t so scary to people, where people can watch a hobbit and can watch a man in a cape, can watch any of those things and not be put off by them. And so I was never scared of the fantasy elements of it at all. That’s what drew me in: the simplicity, but also the brilliance of Philip’s mind. And I don’t think that puts people off. I think that draws people in, and I hope it does in this case.

This interview has been edited and condensed.

Disclaimer: HBO is an initial investor in The Ringer.