When the announcement first came that Saoirse Ronan had been cast as Melanie/Wanderer in The Host, I was confused or disappointed or some such thing. Melanie has a rather detailed physical description, and Saoirse doesn't really fit it; she is also rather younger. Considering that The Host has marked differences from Twilight, but that the media would see greater similarities, I wasn't thrilled about this age difference. None of it really seemed to make sense.
But here are my thoughts now. A Vogue spread (in December, I believe) showed that Saoirse Ronan is one of those people who can look different ages--so, in theory, even if she is younger than Melanie, she should be able to look Melanie's age. Further, it may be useful to have a younger actress: she will easily be able to play Melanie in the flashback scenes and, if a certain amount of time elapses before the second and third movies are made, she will still be young enough to return to the role (assuming that the second and third books will still involve Melanie and that she will still be around the same age).
Last night, I also watched Saoirse in Hanna. Any spy-related film isn't something I'm likely to watch, but I was intrigued by the similarities between Hanna and Melanie/Wanderer. The survival instinct is there, of course. Then in a couple of shots of Hanna walking through the desert, I was giggling at the fact that these could almost be shots of Wanderer. So I've grown convinced that, even if she doesn't look entirely like Melanie, Saoirse can still play her. Considering the way that Hanna doesn't have an exorbitant amount of lines in this movie, yet her face still expresses much, Saoirse will probably also be adept at showing outwardly the internal dialogues of Melanie and Wanderer.
I can never decide if I'm excited to watch The Host or not. I have quite a soft spot for it simply because it takes place in Arizona . . . and it's difficult to see other people's perspectives on something you have that kind of affection for. But we'll see.