When bobbies go very bad THERE is a stark contrast in seeing, in quick succession, two particular performances by James McAvoy. Days after revisiting, with the family, The Chronicles of Narnia: The Lion, the Witch and the Wardrobe, this journalist saw McAvoy in Filth, the adaptation of the Irvine Welsh novel. The two represent an enormous leap in range for the 34-year-old actor. In the past six months, McAvoy's performances as an angry patsy in an art heist in Danny Boyle's Trance, and now as an addled, dangerous detective in Filth, have washed away any preconception of him as an inhabitant of British period films or cute roles (such as his Mr Tumnus, the baby-faced faun in The Chronicles of Narnia).