Suzy Bishop, other than a couple of outbursts of off-screen violence (as one of her classmates put it, occasionally she goes berserk), is not a kick-butt character. Stories are her thing.
When she runs away from home she brings a suitcase full of books and no clothes besides what she has on.
She reminds me of Isabelle in the movie Hugo, another girl who would probably pack books before anything else. And, like Isabelle, she may well grow up to be a writer. That's at least as cool as kicking butt.
Suzy allows Sam to take the lead as they cross the island because of his Khaki Scout training (though there are a couple of moments when she gives him wonderfully uneasy sidelong glances), and then she reads to him during the evening.
Suzy has very specific ideas about what kinds of books she likes: "I like stories with magic powers in them, either in kingdoms on Earth or on foreign planets. Usually I prefer a girl hero, but not always."
And the girl heroes in her books certainly do kick butt ("On the plains of Tabitha, Francine rested. There would be another time for war."). But Suzy doesn't and that's fine. All of her reading has led her astray in some ways, but that's fine, too.
Suzy: I always wished I was an orphan. Most of my favorite characters are. I think your lives are more special.
Sam: I love you, but you don't know what you're talking about.
(The tendency to romanticize orphans in stories comes up in Prometheus also, where David – who only knows about being human from movies – asks Elizabeth, "Doesn't everyone want their parents dead?" "I didn't," is her response.)
And it's Suzy's steady, serious gaze that draws us into the movie. The repeated image in the first few minutes of the film, before we know anything about the characters or the plot, is Suzy Bishop, looking directly at the camera. And she wants to tell us her story, as surely as she reads to Sam every night.
I was pleased to read this comment on the Internet Movie Database, because it shows that this movie has some fans even more obsessive than I am:
When Suzy is reading "Disappearance of the Sixth Grade" at the Mile 3.25 Tidal Inlet campground and continues onto "Part Two" after Sam says to read on, it is just about the exact midway point of the film: the spoken words occur at 46:59, with 46:56 left in the movie. This moment also marks the transition of the film's plot, of course, so Suzy's "reading" also informs the audience of the shift in the movie's tone and direction.
I've watched the film again, and this is right. Suzy is addressing the audience, announcing the beginning of the second half of the movie. Telling us her story.