The usual sign of a book's success is when the movie rights get sold. But often that's when the real hard work begins. Turning a novel into a film is difficult - as Steven Spielberg says, the two art-forms are driven by quite different engines. So what are the problems and how do you solve them? Lynn Freeman talks to director Alison McLean and novelist turned screenwriter Emily Perkins about adapting Eleanor Catton's novel The rehearsal for the screen.