A problem that every writer and filmmaker faces is creating a character that is at least relatable, if not likeable. For director Andrew Niccol, "The Host," the latest adaptation from "Twilight" author Stephenie Meyer, presented a slightly more difficult challenge. How do you relate to a heroine that is actually two characters, an alien inhabiting a young girl and the still vocal mind of the host?

When Niccol spoke with MTV News, he explained that to translate Meyer's literary device to the screen, he had to make a change. Instead of hearing the thoughts of both Melanie, the host, and Wanda, the alien, we would only hear the former and see the latter.

To make that not only convincing on the screen, but compelling, the director needed the right actress, and that's when he found Saoirse Ronan.

"I've watched her since 'Atonement,' and then I saw this movie called 'Hanna,'" Niccol explained. "I thought that this girl can do anything. She can make you cry, and she can kill you. There was something great about that. She has so much complexity. When you've got a character who is an alien who is literally inserting themselves in a human being, that's normally not a good thing, yet you're supposed to fall in love with her, so I thought you really need someone likable. There's something about Saoirse that is impossible not to like, and you need that if you're going to be a body snatcher."

Even for professional actors, convincingly portraying a person taken over by aliens and still resisting mentally requires some help. That's why Niccol rigged a special system to assist Ronan. Before shooting began, the actress recorded all of her internal voice over. That was then played through a hidden earpiece that she wore on set.

"She could literally have a conversation with herself," Niccol said. "There was someone whose job was feeding her those lines. He had to be an actor himself because the timing of the lines was really important. She could hear her own voice with a slightly different accent, because she uses more of a Southern accent for the human being, and she could have this intense conversation, and the rest of the cast and crew couldn't hear the voice in her head."

"The Host" is in theaters now.

Check out everything we've got on "The Host."