Several things have been done to try to ensure that the new show will be a hit too. One was inevitable: “Down Cemetery Road” bears a stronger resemblance, in tone and style, to “Slow Horses” than it does to its own source material. Morwenna Banks, a writer on “Slow Horses” whose background is in comedy, was given the job of adapting “Down Cemetery Road” for the screen, and she has de-emphasized the book’s elements of dark psychological melodrama in favor of a buoyant mordancy.