As a writer-director, Niccol has ample experience with sci-fi, but found himself in uncharted territory handling an existing story. "I’ve never adapted anything before, so I had no idea what to expect. I was a little anxious because she’s had so much success, it would be easy for her to not be normal," he said of Meyer. "But she turned out to be really down to Earth. She was very Soul-like, in many ways because she would be very agreeable even if she disagreed. She says in the novel that the Seekers wear black. And I thought if they have pure motives, I didn’t think I had a chance in hell she’d accept it, but I thought they should wear white. And she said, 'Sure.' That was it!"

While the novel can be helpful as a guide and source of background information, Ronan said that the screenplay is generally the basis of a performance.  She said, "It was more the script for me. I mean, I read the book before we started shooting, but I found that whenever I — it may not always be this way, but with the films that I've done that have been adapted from books, when it's come down to it I'll refer to the script obviously before I'll refer to the book. And that's what I did with this. I mean, it was great to read the book because, I'm sure as you've seen, it's huge, and so there's a lot of detail packed into that story and a lot of backstory for the characters, the world, all that kind of stuff."

In most cases, the author of the book on which a film is based has very little involvement in the production of the movie.  In this case, however, Meyer was actively involved in the development and execution of The Host.  Having the author available was helpful to Kruger, who said, "It's always nice to be able to ask those questions. In this film we don't have a lot of time to see [The Seeker] as a human again, but she definitely had ideas about the human and so she helped put odd moments in the film like that would be odd to an audience, but then at the end you understand it was because her human was yelling at her and she definitely had strong opinions about what the secret should be like."

Selecting Oscar-nominated actress Ronan to play Melanie was the starting point for Niccol in assembling the cast, and the director pointed out just how tricky the character and her arc are, explaining, "Saoirse was first because when you have a character who is an alien who is going to insert themselves into your body, that’s not generally a much loved character, but you have to love this character. You need someone like Saoirse who is really likable. She’s impossible not to like. There’s an inherent goodness to her. Then you have to find, for the guys, you can change the anatomy of a movie – you can change the skeletal structure of a movie, but you can’t change chemistry. That’s why it was an extensive process to make sure those two felt the electricity."

Though its not the central element of the film, The Host includes a variation on a love triangle.  Here, it's something of a love rectangle.  Before being infected, Melanie's love is Jared Howe, played by Max Irons.  Once two consciousnesses are sharing her head, though, Wanderer develops affection for Ian O'Shea, another suitor played by Jake Abel.

Ronan was surprised to find that she was invited into the process of finding actors to portray Jared and Ian.  "Obviously it wasn't just down to me, it was more Stephenie and Andrew's decision and the producers', but I was delighted they asked me to come in because it was important, I guess, that we had chemistry," she explained. "One of the things that I was very adamant about was that, okay, I know that they have to be attractive-looking guys and all, but the most important thing is that they're able to play the characters fully and really bring to life and not just be someone who's a good-looking guy, you know? And thank God we found that with Jake and Max. And I had met them a few years before we started the shoot. I met Max during an audition for something, and Jake was in The Lovely Bones, so I had met them both before and I knew they were both lovely, and when I heard they were coming in I was delighted because I knew at least, hopefully this all works out performance-wise, but they're great guys and I knew we'd all have a great time together and everything."

Embodying the single-minded style of the Souls is Kruger's character, The Seeker.  Since the alien enforcer remains so steely and focused, Kruger found that seemingly superficial qualities were exceedingly important in fleshing out The Seeker, saying, "Yeah, it's weird to say things without intention. But it was a real choice that I guess Andrew, Stephanie, and I made together. Because in the book she's described as relentless and all of that, but she looks very different like she's described as I think short and dark hair and all that and Andrew really wanted the [appearance] to be super pure, sort of Aryan looking I guess. You know very streamlined and very terrifying I guess. So that was definitely something that we created together."

Unlike the antagonist, the hero of The Host is really two very different characters, Melanie and Wanderer.  The internal struggle between the two was basis of this film's appeal for its director. "For me, it’s story first and it was the inner conflict that I loved. Everyone talks about that," said Niccol. "To literally have these two voices that are sparring spirits, fighting over the same body for control who form an understanding and eventually love each other. I thought this is heady stuff. I can say that because it’s not my idea [laughs]. I thought it was a great idea. Also, I like the ambiguity of the movie. Ambiguity is a dirty word in Hollywood. They just want you to have good and evil. The fact that you don’t know if aliens are good or evil. Yes, they take our bodies, but they’ve healed the environment. There are no wars. There’s no hunger. Everyone’s kind and polite. They’re better for the world than we are."

Playing a constant struggle between the human being silenced and the alien in control sounds like a challenge for any actor.  But the lead here was very much up to the task.  "It wasn't too difficult because they're so different, the two of them," Ronan explained. "I mean, it's very much set out in the book and in the script who these characters are and what they're traits are, and I think it's great that Melanie is so feisty and has such a mouth on her. She's a real fighter. She's very, very human and Wanda isn't. She's very kind of serene and positive about things, always keeps her composure. She just doesn't have those kind of very impulsive human reactions that we would to things like being locked in a cell or being taken away from the creatures that you're used to and all that kind of stuff. And I enjoyed playing Wanda a lot because of that. I mean, you really have to keep an eye on when naturally you would react in a certain way. You couldn't with her, you had to keep everything controlled just because it's not in her character to do that."

Meyer's novel already has many, many fans.  The phenomenal success of the Twilight franchise has many of those associated with The Host preparing for a lot of attention from devotees of the author's work, but Kruger is sanguine about the possibility, saying, "I don't know how you do prepare for something like that. Honestly, I'm actually not intimidated by it at all maybe because I've been around for ten years, but also I'm not the object of their...I don't take my shirt off. It's more them, the boys, than probably Saoirse so as a grown woman I can sit back actually and look at them and actually enjoy for what it is and have I guess a bit of a detachment from the reality of those moments."

The novelist has planned a trilogy of books continuing the story that begins with The Host.  Asked if he would be willing to return for a theatrical sequel, Niccol replied, "Yeah, I was drawn to the story and I would love to keep working with Saoirse. Stephenie’s looking at it thinking, 'Maybe I’ll change the second book because of the performance.' It’s that strange situation where the movie might be leading the novel because of her performance… it’s so startling. She’s a bit of a vault. Stephenie won’t tell me what’s going on in the second book. But, it’s supposed to be a trilogy."

"I mean, if the script was right," Ronan said about playing Stryder in a sequel. "I think that's always the most important thing. I wouldn't just want to do a sequel just for the sake of it, you know? If the script is good enough and the story is strong enough to carry on, then yeah. It would be great to carry on."

The Host arrives in theaters nationwide on Friday, March 29th.