Forte and Pullman had a "dream casting session" a decade ago, when she negotiated an option to Pullman's work, to which Pullman added: "I always wanted Nicole Kidman for her extraordinary quality of being warm and cold, terrifying and seductive." He also envisaged Laurence Olivier, circa 1945, as Lord Asriel, a role eventually taken by Craig because Olivier "wasn't available".

*****

Although Lewis's and Pullman's religious beliefs are radically different, Lewis's Chronicles of Narnia shares much with The Golden Compass: talking animals, witches and child protagonists travelling to fantasy worlds. But Pullman's coup is all his own: the daemon, pronounced "demon". In The Golden Compass, almost everyone has a daemon, an animal that is another version of the person to whom they belong. They feel the same things, mentally and physically. The daemon is an idea borrowed from Socrates, who believed he had a divine presence within himself. He called it a "daimon" and it would warn him if he was about to do something bad.

*****

For Pullman, the daemon was also a great literary device. As he told the Oxford audience: "The moment I thought of daemons was on the 16th draft of the first chapter. Before that I had to tell the reader what Lyra was thinking. I realised then I didn't have to explain so much. You don't need exposition. Exposition kills the flow of the narrative.

"The best advice ever given to a writer was by Raymond Chandler, who said, 'When in doubt, have a man come through the door with a gun.' This works. If you're stuck with the story, it moves it on like nothing else." Lyra's daemon, Pantalaimon, "was my man coming through the door with a gun. It was a wonderful moment when I realised that."