For horror film fans it looms large like the Lincoln Memorial or Washington Monument. An official city plaque announces that a precipitous stone staircase in Georgetown, Washington DC, will forever be known as the Exorcist steps. It marks the spot where “Father Karras [the actor Jason Miller] plummets the seventy-five steps to his death” at the movie’s climax. Joggers who work out on the steps might get some company on Tuesday from Exorcist devotees savouring a milestone: the 50th anniversary of the release of a movie that prompted reports of fainting, vomiting, epileptic fits and audience members charging the screen...