When Sally (Toni Collette) returns home and looks into a reference book she looks at an article on Leopold and Loeb. This was a famous case from the 1920s where two fellow students from the University of Chicago decided to commit the 'perfect murder'. They were easily caught but were spared the death sentence after a highly-regarded defence from Clarence Darrow.
The "Maraclea" is a Knight's Templar fable from thirteenth century France which when seized on by the adolescent protagonists of this movie leads to murder and trophy-ism. Writer-director Gregory Read elaborates, as the fable goes, "Maraclea was betrothed to a Knight Templar, but died before they could wed. The Knight was so tormented by his loss he opened her grave and made love to her body. Exactly nine months later, he was summoned to the grave. On opening her coffin, he found her remains had been moved, her head had been placed below her pelvis, to sit on top of her thigh-bones which had been crossed. A voice told him to guard the skull as it would be the 'bringer of all power'. The knight took the skull and found, with the skull in a grail cup, he could defeat and slay all enemies. The idea was that the skull can possess a lot of power, but there has to be a certain process to obtain that skull. I felt this mythology would be of great interest to my psychopaths."
Writer-director Gregory Read said of the casting of the Nigel character: 'For Nigel I was looking for someone dark and quiet. I didn't want awkward and weird. I saw Tom (Sturridge) in 'Being Julia' [2004] where he plays the model son. I thought, here is a boy who could quite easily play evil or dark, but why don't we just play him as a recluse and let the evil grow out of this?".
In the meeting of the two central male characters, Alex (Eddie Redmayne) and Nigel (Tom Sturridge), another element of writer-director Gregory Read's research gave the story a dynamic twist: Gestalt Psychology. Read said: "Gestalt means that the whole is bigger than the sum of the parts. It is usually used in psychology for the good, not the bad, when groups of people come together to create something bigger. But when psychopaths come together, they may create something unwanted. Two people who are pretty well incapable of doing it by themselves, between the two of them, one manipulating the other, suddenly find themselves able to kill."
The schoolboy leads were always going to be a tough casting call. Alex, who presents the psychological puzzle the audience must unravel, especially so. For Alex, writer-director Gregory Read felt the story required charisma and unconventional good looks, not to mention the kind of intelligence that prompted Read to think of the character as "the master puppeteer". Read found these in the then young theatre thesp and then recent Cambridge graduate, actor Eddie Redmayne. Read said: "I saw him in a play called 'Hecuba' in Covent Garden and he was incredible...I wanted to find someone who had the eyes. The casting was all about eyes."