I'm only guessing that the movie of How I Live Now is better than the book it's based on; I haven't read the book, just a summary. Which hardly tells me how well Meg Rossof carried off her characters and her action; it could all be great. But the book's got twins, and telepathy, and what looks like a lot of clutter which the movie wisely shaved away until you've just got no more than you need to pull in the audience and rip them apart for a couple of hours. Rossof seems to have thrown a lot of cool stuff in to make the book more exciting and have lots going on; the movie pares it right down to the basics, discarding characters and themes and sideshows to give you one exhausting slice into the heart of the story.
I've said often in these posts that you don't need to put the world at stake; you just need to make the characters real enough that they become the world. How I Live Now has got elements in common with The Hunger Games, including a teenage female lead, a little girl for us to worry about, a cool brooding hunk for the girl to worry about, a civilisation destroying war and a whole bunch of trying to get by in a wilderness. But The Hunger Games puts the lead right in the centre of everything that matters in the world, while How I Live Now puts Daisy right on the edge, completely adrift in a world which never gets a full explanation. She's just going through the random stuff at the edge of an apocalypse; the end of the world just happens to be the end of her world as well. In a more American outlook on things, she'd be the plucky survivor who would find the amazing secret which would end the war, switch the power back on and make it rain unicorns, but Daisy starts out as a neurotic mess and doesn't get hugely more competent with time. She scrapes by, and no-one watching would fool themselves for a second that they'd do any better. Daisy grows and develops, but it's the growing and developing we all do; from barely getting by as kids to barely getting as adults.
There's a slightly weird overlap thematically with Kevin McDonald's last film, The Last King of Scotland, which was also about someone coming into a brewing war zone without much of a clue and finding their way back out of it more by luck than judgment, but How I Live Now doesn't have an Idi Amin, just the chaos of a country falling apart in slow motion. From that point of view it's got much more in common with Cuaron's Children of Men, on what looks like Children of Men's doughnut budget; it's a good looking movie, but it's determinedly small scale. It goes off-road as soon as Daisy gets out of the airport, and from their on out it's doggedly rural and there are never more than a few people in the frame.
McDonald has thus doomed himself to live and die on the strength of the players. So he hired Saoirse Ronan, leaving himself with nothing to worry about. Ronan can sell just about anything, but more importantly she's got the kind of star quality which lets her collapse into a very small space and draw you into after her. When your whole movie rests on one character, nothing but the best will do. Ronan exudes attitude from the moment she slouches onto the screen and through passport control, but at every swagger she lets you see how precarious that confidence really is.
It's an unenviable day out for the rest of the cast, playing teens and pre-teens up against someone who got an Oscar nomination for her first big day out at the age of 13 in Atonement. They just seem to have taken a deep breath and gone for it, with pretty fair results. I'm not sure what anyone could have done with Edmund, Daisy's crush; being dishy, manly and taciturn doesn't give an actor much to work with. Isaac and Piper give their actors more of a chance, and they pretty much nail it, with the rather wonderfully named Harley Bird doing a great job as Piper. Piper could have been a tiresome fey moppet plot token, but Bird makes her into a credible temperamental little girl.
Still, it's a gruelling movie. McDonald doesn't show much violence directly, but suggests it constantly, showing us the aftermath, or flickering glimpses by firelight of things best not seen. It's a subtle, enervating way to show that war is hell even for the people on the edges, but it's no-one's idea of fun.