Warning: Spoilers ahead for "The Handmaid's Tale" through episode three, including details from the book not yet featured in the show.

Hulu's new drama series "The Handmaid's Tale" premiered on April 26. The show was adapted for TV by showrunner Bruce Miller, who was a longtime fan of the Margaret Atwood novel upon which it's based.

Among the many changes made to the original book, one of the most noticeable is how two characters — Serena Joy and Commander Waterford — are played by much younger actors than expected. 35-year-old Australian actress Yvonne Strahovski plays Serena, while 46-year-old Joseph Fiennes was cast as the Commander.

In the book, both Serena and the Commander are presumed to be middle-aged or older, with arthritis and graying hair. In an interview with INSIDER for our new "Showrunners" podcast, Bruce Miller explained the reason for the younger characters, in particular Serena Joy.

"That was another big decision and certainly something I talked extensively with Margaret [Atwood] about," Miller said. "Serena Joy in our story is younger. Long before we started casting, I was thinking about that. When I saw the original movie, it was Faye Dunaway, who was quite old at that point. In the book, they don't name the age of Serena Joy, but she's seemingly elderly. She has a cane, she's got arthritis, she's got gray hair. I felt like in the novel there's only so much of the dynamic between Serena Joy and Offred that you're going to see, but in a TV show it's going to go on and on and on hopefully for years."

Yvonne Strahovski as Serena Joy. Hulu

For Miller, it was a more interesting dynamic between a wife and her Handmaid — a women who has been forced into procreational servitude under the assumption that the wife is infertile.

"The element that was missing for me was the direct competition between the two women," Miller said. "Because with an older Serena Joy, she wants something so badly that she felt like she should have had in the past, but she's past that point now. She's older, and she's using this young woman to try to get that. I felt that it was a more active dynamic if Serena Joy felt like this person was usurping her role not only as the reproductive object of the house but gradually taking away the wifely duties, the intimate duties, the romantic, sexual duties."

Elizabeth Moss plays Offred, the Handmaid assigned to Serena Joy. Hulu

"Then something I didn't expect to happen is that Yvonne [Strahovski] and Elizabeth [Moss] are close," Miller said.

Strahovski and Moss are just one year apart in age, which creates a whole new potential for relationships between them. "You get that little vibe once in a while that in another situation they could be friends," Miller said. "It is the creepiest thing."

And the new layers of their relationship and potential for new scenes in the show doesn't stop there.

Warning: Spoilers from the books below.

Serena Joy often towers over Offred. Hulu

Having a younger Serena Joy means Miller has more narrative avenues in the future if a second (or third or beyond) season is greenlit by Hulu.

"At some point you find out Serena Joy is not sterile," Miller said. "If it's the Commander [who is sterile] and Serena could be fertile, that opens up a whole lot of doors for us story-wise. When you work in TV, you're always trying to think of just filling up your bag with tennis balls because you don't know when you're going to have to play tennis with them. You always want all sorts of interesting stuff to be happening."

As Miller said, aging down Serena Joy was already the plan before they started auditioning for the show. Then Strahovski came in, and cemented the idea in Miller's mind.

"She was so astonishing in her audition," Miller said. "She made me feel sorry for Serena Joy, which is seemingly an impossible task. I felt bad for her. She was so wonderful and terrifying. And she's quite tall, so that works really well with Lizzie who is more small. Serena Joy wears heels and Lizzie doesn't. To have this towering viking standing over her ... she's physically intimidating."

Their power dynamic is reflected in their height. Hulu

Serena Joy has a particularly physically aggressive scene with Offred at the end of episode three. After believing Offred was pregnant and treating her with over-the-top kindness, Serena Joy loses her temper when she hears that Offred's period started. Serena throws Offred to the ground before diving to the ground right next to her face, making sure her fury is understood.

Miller said that scene was improvised by Strahovski.

"She said, 'I think I'm going to try something on the first take,'" Miller said. "Yvonne [Strahovski] just got right down in her face and it's terrifying. It's such a great moment. Imagine if you were the cameraman not knowing that was going to happen, which he didn't. Then he's like kneeling down on the ground shooting and all of a sudden Yvonne [Strahovski] comes barreling in. It was amazing."

No one knew Strahovski was going to throw herself on the floor. Hulu

The improvisation paid off.

"The [footage] we used was the first take that we had," Miller said. "God bless our cameraman. If you see the shot, it does not shake when she comes in. She dives at you like a linebacker, and he doesn't go, 'Ah!' — which is what I would do."

That scene is one example of why having a younger Serena can pay off. The Serena Joy from the books, in pain with arthritis and using a cane, wouldn't have been able to dive to the floor and scream in Offred's face.

"I think that's the vibrancy that you get from having a younger [Serena]," Miller said. "Just the physical intimidation. Yvonne is a whip-strong woman. Lizzie [Elizabeth Moss] is also quite strong. The two of them together, you feel like, 'I'd love to see them go toe-to-toe in a cage match.'"

You can read about more changes Miller made to the original story here, and make sure to subscribe to "Showrunners" to hear the full interview with Miller when it premiere on May 1.