The prison seen in the film is actually a combination of three filming locations: the exteriors are that of the Meaux prison, some 40 kilometers east of Paris, the entrance corridors are part of the Bibliothèque nationale de France in Paris and the other interiors (the cell, the visiting room) are studio sets.
Fred Cavayé and Guillaume Lemans didn't do an enormous amount of research as they didn't want the audience to forget that the hero in an ordinary person, an amateur, who doesn't really know how to proceed.
Diane Kruger insisted on meeting with Fred Cavayé before accepting the part, as she felt that the character of Lisa wasn't fleshed out enough. As such, Kruger and Cavayé spent several months developing the character which had a significantly smaller role in the original draft.
All the prison scenes and the studio interiors were shot in chronological order as Fred Cavayé felt it would be important for the actors and the evolution of their characters.
Klaus Badelt came onto the project at a late stage, and watched the film with Fred Cavayé to see where music was needed. Though Badelt felt the film already worked without music, Cavayé found that his work, which was recorded at London's Abbey Road studios, elevated the intensity and the emotion of the scenes to new heights.