'Dune: Prophecy' episode 3 reveals Tula Harkonnen's shocking backstory. Emma Canning breaks down that devastating twist.

• Emma Canning plays young Tula Harkonnen in "Dune: Prophecy."
• The third episode dives into the Harkonnen sisters' backstory and the pivotal choices they've made.
• In an interview with BI, Canning broke down the major reveal about Tula's past with the Atreides.

Warning: Major spoilers ahead for "Dune: Prophecy" season 1, episode 3.

After a fatal series premiere, "Dune: Prophecy" has claimed more lives — but rather than burning up from the inside, these victims died quietly.

Episode three, "Sisterhood Above All," dives back into the Harkonnen sisters' childhood. Before they were Reverend Mothers, Valya and Tula grew up on the cold-weather planet Lankiveil when House Harkonnen was scraping by. Their brother, Griffin, is killed, and Valya, who believes Vorian Atreides to be responsible, swears vengeance against the family that engineered the Harkonnen family's fall from grace.

It's not Valya who exacts that price, however. It's her younger sister Tula, played by Emma Canning as a young woman and Olivia Williams as an adult. To carry out her and Valya's revenge, Tula seduces (and falls in love with) Orry Atreides. In turn, she poisons him and almost his entire family on a hunting trip the night that he proposes to her.

Williams broke down the Atreides massacre with Business Insider, including Tula's decision to kill her fiancé — and her choice to let the youngest Atreides live.

Tula seems to be wrestling with how to address Orry after she's already set the Atreides massacre in motion. How did you approach that tension and her motivation after he proposes to her?

In that sequence, from the moment she hands over that bucket of poison at the fire, it really was about having a plan, being faced with an obstacle, and having to change your plan. I think she walks up to the house — he should go down, join them, smear himself with poison, they'll go to bed, and so he'll be with them.

He's had a really tough day, he had to put down the horse, and that wasn't something she predicted. So then it becomes a thing of, "Okay, well, he's not going to leave the hut. What can I do to keep him in the hut? How can I stop him from going outside?"

I think then that's what the game plan becomes. She opens up to him, she says yes to his proposal. All of those things, I think the undercurrent is, "I can't have you leave right now."

As a viewer, there's a sense of resolve but also reluctance on Tula's part. How did you approach those more difficult, almost contradictory emotional beats?

Contradiction is really helpful as an actor. Richard Lewis, our director, and Alison Schapker, our showrunner, had given me this huge contradiction in really being very clear that this is a love story: She's in love with Orry, and she is falling deeper and deeper in love with him. So I have this major pull between my love for family and my loyalty, and then my growing love for him and my growing loyalty to him. So the complexity of the push and pull is line by line.

It really is dependent on how Milo Callaghan, who was playing Orry, would deliver certain lines. We were really lucky in prep that we got to know each other. We had to do horseback riding. We had about two weeks of that, so we were good pals by then, which was really lovely.

We shot this sequence very isolated from the rest of the scenes that I played, so Milo and I kept being like, 'Are we doing a short film with Richard?' It felt so intimate. It felt so small. I think we had to keep reminding ourselves, "No, this is 'Dune.'" But that was really lovely in that it really took the pressure off. They're just really good scenes and really high stakes.

Tula eventually does kill Orry, but she decides to let Albert, Archie Barnes' character, live. How did you rationalize her decision at that moment?

We had done this scene where she gets to kind of see herself in him. He is the youngest member of the Atreides. He is passed over, brushed into the corner, and develops a relationship with Tula that I think echoes her relationship with Lila later on, in Tula being the older sister that she always wanted and didn't get. I think by the end of the massacre, when Albert appears, she's forgotten that he's there.

To be honest, I think that the decision to inject Orry is one of self-protection and survival. I think she's very fearful of her own safety once he sees what she's done. And I think Albert doesn't pose a threat. It's kind of, "So long as I survive, you don't hurt me, you can go."

When Tula goes back to the Harkonnens after this, they won't even let her take accountability for it because they blame Valya. How did that sort of impulse for them to frame Tula as the "good" sister affect how you approached her?

I do think the sisters are constantly being contrasted. I think they are very different, I think they both like their differences. I think they are both proud of their differences and also really envy the other's differences.

Tula being the good sister — I heard a phrase when I was prepping of like, "The youngest child raises themselves," and that's kind of what I brought through. I didn't see her so much as being good, but I just thought she was never causing any friction. She did her own thing, glided through.

Something I held onto was when Valya and Tula are discussing the acolytes earlier in the season, and Valya describes Lila as a little lamb lost in the woods, Tula really resonates with it. She's like, "I was like that." And that was an image that I knew was important. I knew I had to try and embody that, because Tula needs to be able to recognize that in herself later. I also had in mind "a wolf in sheep's clothing," so I wanted to braid that through.

I know that you and Olivia Williams didn't film together at all — what did preparing to play a younger Tula look like on your end?

I had about two months' prep before stepping on set. I'm Irish, so I'm not speaking in my own accent, whereas Olivia is, so I started there in terms of just specifying intonation and really listening to her podcast, being kind of sneaky and sly. I didn't tell her that I'd done that, though.

Then I got to Budapest, and Olivia really kindly carved time out of her crazy schedule, and we met and talked about Tula. She'd already begun work at that stage. I think once you begin playing a character, the sense of knowing them becomes much stronger. It feels a bit more innate because you're living within them.

But even still, she was incredibly generous having already been working on this character. She didn't stake a claim on Tula in a way that I am just really thankful for.

When we were talking, it was almost like Tula was sitting on the table in front of us and we were both chipping in. She obviously had to do such a huge amount of imaginative work. She doesn't play the younger Tula scenes, whereas I get to live through them and react within them. She has to also track what that does to a person, and how to they process that.

"Dune: Prophecy" airs Sunday nights at 9 p.m. ET/6 p.m. PT on HBO, and is streaming on Max.