As far as movies go, How I Live Now isn't bad. It's not the best movie I've seen this year; the dialogue is sometimes weak and the romance is contrived and too simple to drive the character through the challenges she faces, but—it does some interesting things that one rarely sees in a Hollywood production. (It's a British production, in fact.)
It begins as a basic story about an antisocial American girl visiting the UK against her will. She falls in love with a nice British boy against the nice country backdrop. Then, out of nowhere, bullets are flying, the bomb is dropped, explosions, radiation, contaminated water, forced labor, digging through corpse-piles to find family members, rape, murder, etc.
So, how does that work?
The change isn't exactly out of nowhere. The director and writer used effective foreshadow in the form of military presence in the airport, increased security screenings, bombing news in the background, conversations, etc. This all added a subtle layer of tension to the film that otherwise would have led to a jarring and unpalatable change in pace, conflict, genre, and style.
It's an unconventional and risky technique. Some won't like it, but I found it refreshing, despite the unnerving and unexpected bursts of violence and intense imagery. I like that it skipped the usual CGI-laden scenes of apocalyptic destruction and told the story on the ground, from a personal perspective.
In storytelling, there are often a multitude of changes that can turn a reader off. Beloved characters die, for example. Settings change without warning. Tone might change, as well as theme or pace. Foreshadowing is one technique a write can use to prepare the reader for that turn, and even create tension as they wait to see something they dread unfold.
In the case of How I Live Now, it made one critical difference—without that foreshadowing, I wouldn't have made it to the bomb. Even as it is, I was about to change to something else, but I kept watching a little longer because of those little hints at something awful just up the road.
Parenthetically, it is fair to say that perhaps it wasn't the best approach. The writer could easily have chosen to begin later in the story, since the romance was poorly contrived anyway. The tidbits about the main character's rules and compulsive nature could have been covered in-story or through unfolded backstory, as could the romance element.
Writer of speculative fiction. Current work in progress is an untitled dark fantasy novella.