Explore "The Technique of the Mystery Story" by Carolyn Wells on Textopian. Read with annotations and highlights.
Fog, though not so much of a factor here, is incessantly used in English detective stories. It is so obviously a convenient device for concealing crime, that it is the foundation of Richard Harding Davis's "In the Fog" and also of "A Mysterious Disappearance," "Big Bow Mystery," "The Masquerader," and scores of others.
Another hackneyed device is the secret panel in the wall, which slides open by pressing a hidden spring. This was overdone in sensational fiction, before Detective Stories began, but was seized upon as a valuable device for Mystery tales. But it is easily suspected, and in unsatisfactory in modern settings.