But let's talk about the romance first, and teenage star Saoirse Ronan's big-screen kiss.
"The first take is quite awkward and you are a bit nervous about it because no matter how close you are to the people that you are working with, when they are this close and, you know, you're about to snog each other, it's a different story," she said. "But then you get kind of used to it and you just get on with it. And the thing is that you have to do it."
Ronan plays a tough teen who's still alive inside even though an alien is what you see on the outside.
Max Irons plays her confused love interest. However, thanks to extra rehearsal time, everything made sense.
"You know, we got two weeks of rehearsal, which William Hurt insisted upon, where we all sat around and decided what the themes, the underlying themes were, which is unheard of," Irons said. "But the producers said, 'Yep, it's important and we're going to invest in that.'"
Diane Kruger plays a relentless alien seeking out any left on the planet who still might be human. It's not a happy role.
"Yeah, no, not many smiles," Kruger said. "It was interesting, you know, she's a little Stepford-wifey, but sort of terrifyingly pleasant, right? Like everything is, that was really interesting to play."
Co-star Jake Abel knows the movie is complex but thinks young audiences will understand it and enjoy it.
"They let him make a film that burns slowly, and that tells a really interesting story, and doesn't just try to cram everything because they think kids have a short attention span," Abel said. "I mean, anyone who's read this book, it's over 600 pages. They obviously have an attention span and they can handle character development."
"The Host" is rated PG-13 and it's probably best for a teen audience.