The film makes quite a few changes to the book. In the novel, Miranda's tyrannical nature is more obvious. In the film, they decided to tone her character down. She's still as domineering, but they thought it would be funnier if Miranda remained calm while making impossible demands. Nigel is only a minor character in the book and a flamboyantly gay stereotype. His role is expanded in the film, and while he is probably gay, Stanley Tucci's portrayal of it is a little more subtle (glancing at men at a fashion shoot). Also in the novel, Miranda is British and Emily is American. In the film, they're swapped around. Emily Blunt insisted Emily should be British. Finally, Lily's accident in the book becomes Emily's. Lily becomes an alcoholic after watching the effect Runway has on Andy, but instead of being hit by a taxi like Emily, she drives into one in the book.