Preests statement - "If a god is willing to prevent evil, but not able, then he is not omnipotent. If he is able, but not willing, then he must be malevolent. If he is neither able or willing then why call him a god? Why else do bad things happen to good people?" - is almost directly lifted from Epicurus, who is credited with first expounding the problem of evil. David Hume in his Dialogues concerning Natural Religion (1779) cited Epicurus in stating the argument as a series of questions: "Is God willing to prevent evil, but not able? Then is he impotent. Is he able, but not willing? Then is he malevolent. Is he both able and willing? Whence then is evil?"
Initially, Ewan McGregor, Paul Bettany and John Hurt were set to star, but the film's schedule was thrown when McGregor broke his leg in a biking accident and was forced to drop out. Eva Green remained the only member of the original cast.
Near the end, when Preest/David is in Emilia's London flat, he asks her what her faith/religion is, to which she replies that she is not religious. In real life, Eva Green is Jewish.