The director, Kevin Macdonald, had originally intended the cast to be all unknown or amateur actors and was looking for an American girl, around 16 years of age to play Daisy, but cast Irish actress 19 year old Saoirse Ronan instead, after her audition, reading a scene which left them in tears.
Kevin Macdonald chose to film the first half of the film with a hand held camera to give the paradise-like countryside home a sense of humanity, as though the camera was alive and breathing. The second half of the film was shot in a more steady and smoother style to make the war torn countryside more sharp and unforgiving, as though the camera was mechanical.
Aerial shots were filmed with a Canon EOS C300 strapped to an opticopter/drone: "We were a small-budget film; we couldn't afford a helicopter and we couldn't wait for the weather," said director Kevin Macdonald. "There were a few shots in the film which were filmed with these adapted toys, and they have to have a lightweight camera. The Canon [C300] is at the top end of the weight they can manage, but we got some great aerial shots, which we use quite often in the film. That was a real bonus that you couldn't have got with any other camera."
In the book, Edmond and Isaac are twins, both 14 years old; and they have an older brother called Osbert, who is 16. He hangs out with his own friends and is not a part of the tight group of siblings Daisy becomes close with. In the movie Edmond is older than Isaac and is the oldest sibling. Also, in the movie Edmond communicates with animals - which is Isaac's trait in the book. In the book Edmond can read people the way Isaac can read animals.
"During pre-production, before we actually started shooting, [cinematographer] Franz Lustig went around and shot [with a Canon EOS C300] lots of little bits of birds, landscapes and details of things," said director Kevin Macdonald. "This is something you always want to do on films because you can see a beautiful sunset and have the flexibility to pull out a little tiny camera and shoot something. That stuff has all made it into the film. Franz could shoot this on his own." Lustig said: "It was a one-man show! Me, my backpack, three lenses, and maybe a tripod if needed. I could do images for the film and they cut in beautifully. You don't need a whole crew. We also tested some things in moonlight,(...). That was the original reason we wanted to try this camera for its low-light abilities."