Andrew Niccol (screenwriter and director) really has not done many interviews for The Host.  He did not participate in the junket in LA, but we were able to speak to him briefly on the red carpet.  Crave Online (source) has an in depth interview with him where he addresses the two parts that Saoirse Ronan acted out, the ending, the aliens, and the overall look of the film.  Excerpts are below, but I highly recommend you go read the full interview.

Stephenie Meyer is coming off of all these other adaptations of her books, and they’ve done so well. Was there more pressure or less pressure on you, do you think, because of that?

I’m so, probably, naïve that I didn’t think too much about it. Everyone would say that, but I’d go, I have a job, so I’m just going to make this as good as I can. But I’ve never really felt the expectation. The only thing, I have more ephemeral creative pressure to honor the book and the story of the book, rather than commercial pressure. It’ll do what it does.

I have not read the book, my apologies. Has there been a lot of major changes to make it filmable? Because a lot of it’s in the character’s head…

Right. Well, that was the big change. In the book you read conversations, and it’s two sets of thoughts, and as compelling as Saoirse Ronan’s face might be that’s not very cinematic. That’s radio, if I’m just going to photograph your face and hear two sets of thoughts. So I made the decision early on that I would hear the host’s voice – Melanie’s voice, the human voice – in her head, and see [Wanderer] speak, so that even if she had to hide the fact that she was having a tense conversation with herself you would have something visual to hold onto.

Did Saoirse record all of that in post, or did you record it earlier so she could react to it?

We recorded her whole performance before we shot. I added other things later, but I had this earpiece that I put in her ear, a hidden earpiece, so that she – and there’s one technician, whose job was to basically feed her her own lines, her own voice – so she could hear it. But none of the cast or crew could hear it in a scene, which was very odd. And I could hear it. But she would have these intense conversations with herself wandering through the desert, and her lines are being fed into her head! Like some babbling homeless person!

Everything the aliens have is bright and silver. What was your thought on that, were you thinking about a certain movie?

No, I was thinking that the design philosophy should come out of the philosophy of the Souls, and that the Souls… Most of them are dressed modestly. There’s no signage, of course, because there’s no competition, and they would never want to be flashy unless they had to stand out, like the Seekers. The chrome from the Seekers comes from the chrome of the creature itself, so that’s why they have an affinity for chrome. It reminds them of their natural state.

Regarding the aliens in their own “skin:”

You’re closer. [Laughs] I wanted them to be ethereal. I didn’t want them to be a fearsome creature. I wanted them to be quite fragile, in fact. You could crush them. They emanated so much light in fact that you cannot tell their true shape, which I thought was interesting. It keeps a mystery to the creature. Even though you’re looking at it you can’t really see it, it’s like our ideas sometimes of God. You can’t really look at [God], it’s just a light.

There’s a couple of questions I wanted to ask you about the ending.

Sure.

In no particular order. It seems like a set-up for a sequel.

It is supposed to be a trilogy.

So you’re letting that…?

I’m allowing for that next phase to go.

Would you want to come back for that one, or do you feel like this was enough for you?

I want to see what the book is, first of all, because Stephenie’s supposed to be writing… There’s The Host, and then The Seeker and there’s The Soul, is the trilogy.