As in so many mystery novels, Roger Ackroyd is set in a small, isolated community where everybody knows everybody else. The isolated, close-knit setting 1) suggests that the criminal is someone who everybody knows, and 2) creates a paranoid, suspenseful mood, since the criminal is hiding a big secret from their neighbors. It’s also telling that the owners of the two most “important houses” in town are also the two main victims of the novel.