Dr Alithea Binnie, played by Tilda Swinton with a northern accent and school-marmish glasses, specialises in "narratology", which sounds like an academic field made up for The Da Vinci Code, but is actually the study of narrative structure. She is at a conference in Istanbul when she buys a stripy glass bottle from a shop in the Grand Bazaar, and then polishes it with her electric toothbrush in her hotel room. Before she can say "Aladdin", a djinn appears, as played by Idris Elba with pointed ears and furry legs – and once you see Elba towering over Swinton, with his rumbling voice and his sweet mixture of authority and self-doubt, you can"t picture anyone else in the role.