Chiwetel Ejiofor and Craig Zobel on Z for Zachariah‘s Surprising Ending

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Correction appended, Aug. 31

What would you do if you thought you were the last person on Earth, and then someone else came along? That’s the question faced by the characters in the new Craig Zobel-directed movie Z for Zachariah, in which Margot Robbie plays Ann Burden, a young woman who’s been protected from nuclear fallout by the self-contained weather system of the valley where she lives alone—until John Loomis (Chiwetel Ejiofor) shows up. And unlike Ann, a devout Christian, Loomis is an atheist and a scientist—he’s been protected from radiation by a high-tech suit—and views their situation in practical terms. Just when it seems they may be ready to take on the work of repopulating the planet, scruffy coal miner Caleb (Chris Pine) shows up, proving that three’s a crowd.

The film is an adaptation of the 1974 novel by the same title, though the character of Caleb was invented for this version; the addition complicates every aspect of their existence, from religion (Caleb, too, is a Christian) to sexual tension (Ann now has a choice of mate), making their valley a microcosm of human relations.

TIME caught up with Zobel and Ejiofor ahead of the film’s release on Friday to talk about small casts, the sci-fi genre and the film’s surprising ending.

TIME: What drew each of you to this project?

Zobel: I was drawn to the idea that it was a way to talk about relationships. It has a moment of people who are being individuals, and being alone and living with that, and then having to be with another person—even in a platonic way, just having to share a house with another person changes your life slightly, you know?—but then of course any romantic feelings… changes things. Adding a third person, it becomes a community.

Ejiofor: I thought it was fascinating for much the same reasons. I’d also been a huge fan of Craig’s film Compliance, which was a really fascinating film. Even though it’s very contained [because it’s] set in a fast-food joint, it had an epic scope and a dynamic quality to it—the discovery of characters and the nuances of language and personality. And to get into the interpersonal relationships of a two-hander and then into a three-hander, being able to ratchet up the dramatic tension just on the basis of personality—I thought, as an acting exercise, it was pretty exciting.

Had either of you done any post-apocalyptic reading in preparation, besides the book this is based on?

Zobel: In my life I have. I’m a big fan of Alas, Babylon, which is kind of in the same vibe of being a realistic post-nuclear situation.

Ejiofor: I hadn’t really looked at it in terms of novels, really, but the [cinematic] sci-fi reference points are always quite strong. You [Craig] were talking about that movie The Quiet Earth. I was thinking about it in terms of the films that I’ve seen that have a minimal amount of characters. The ones that spring to mind are Dead Calm. Then there’s that movie Sleuth with Michael Caine and Lawrence Olivier.

How was working with such a small cast different from other movies you’ve worked on?

Zobel: The more I think about it, it’s still the same work. The plus is you get to know each other enough where the communication is a little faster.

Ejiofor: It’s interesting. I don’t know 100% if that’s right. There is a point that we got to where we were actually communicating at a very high rate. I remember, there was a conversation we had outside the trailers, and it was the four of us standing up, talking in a kind of a huddle. By that point we had such a rapid shorthand that there was this quickfire session that actually went on for quite a while, all of us pinging the ideas we were thinking about that scene. It’s very hard to imagine that occurring, actors and director, without ego—to be able to build that level of conversation, of trust, engagement, is quite rare. It required all that time and isolation.

Zobel: That’s true. And I’m not sure that that scene was, frankly, written as good as it could have been, and I like it in the film—it’s one of the dinner table scenes. I think it’s a strong scene in the movie, but I don’t know that it would have survived the edit if we hadn’t done that.

How did the religious aspects of the film come together?

Zobel: It’s baked into the story from the novel on. I didn’t want to make it about that first and foremost, but it’s a tribe we all do or don’t join. The interesting thing is [Ann] truly believes, and I don’t necessarily have that strong a faith, but there is a part of me that when I see people who really, truly believe, it’s fascinating to me. That does help them, and it’s something that I don’t have. If I were in her place, I would probably not feel the same way. More than anything, [it’s] essentially a level of politics that they can play.

Chiwetel, your character is more science than church. Personally if you were in this world, would you be more on the science side or the church side?

Ejiofor: I think it would be a transition from atheist to agnostic. Loomis is definitely an atheist, and cannot and will not shake that—even in the face of his minoritization when Caleb turns up and they’re starting to bond over their religion, at which point he’s completely outmaneuvered. Loomis’ close-mindedness to all that ends up not being very helpful to him than a more broadly agnostic approach might have. That’s probably where I would have ended up.

Obviously you didn’t pick the title, but who or what do you think is Zachariah?

Zobel: In the book, the idea is that it’s kind of like a reference on “A is for Adam” would be the first man—this certainly has an Adam and Eve thing going on—and Z is for Zachariah, he’s the last man.

Ejiofor: What is the character Zachariah? I can’t remember now.

Zobel: In the Bible? Gosh, now I can’t remember either. It doesn’t correlate quite correctly.

So, I have to ask: Did John drop Caleb?

Zobel: I think you know.

I think he does…

Zobel: Yeah. I feel like it’s heavily hinted at.

Definitely, but I did leave wondering if maybe he did decide, It’s too crazy, I’m just gonna hit the road.

Ejiofor: That’s not a terrible thing to think. I think it’s slated one way, heavier in one direction than the other.

Zobel: Sure. Because you don’t get that moment, you’re allowed to have hope.

Do you think Ann knows?

Ejiofor: She’s gotta be deeply suspicious either way. The real thing is what they can rebuild—and if they can. Or is there a point where she does drive him off the land. Is that in their future? Or is there a future in which they actually figure it out?

Zobel: It certainly isn’t superfluous why Caleb isn’t there anymore, but certainly the fact that he’s gone and Loomis is by himself is enough of the problem for her. I think it’s a different story if you fast-forward two days after the movie ended to, like, six months after the movie ended—might totally be different stories.

Correction: The original version of this story misstated the title of a post-apocalyptic novel by Pat Frank. It is Alas, Babylon.