Your characters are the appetizer when setting the scene for mystery ... but locale is the entree
You can go a long way in writing a mystery without fleshing out the main characters. But try doing the same with the locale--the story's physical scene--and you'll soon wind up in a boatload of trouble.
Readers don't mind having to wait awhile to get all the information they can possibly get about the people in a mystery. They're used to having characters developed slowly, a little at a time, as events take place and the story unwinds. Characters, after all, show the reader what they're made of by reacting to action and relating to specific events--and those things take place in due time. It's hard to know just how selfless a hard-boiled P.I. is until the reader witnesses his leap off a towering bridge to save a hapless suicide victim.
But readers can and do object to being kept in the dark for too long when it comes to setting a mystery's scene. And for good reason. The scene sets the stage for the element of mystery and triggers a readiness in the reader to be, well, mystified. Does the story take place at midday in a bright, sunny park filled with tittering children and chattering old maids? Or does it unfold at midnight in a rainy alleyway where the only sign of life is a stoop-shouldered ghost of a man shuffling slowly along ... and the only way out is up? It makes a difference. Whereas the one creates within the reader little speculation about mysterious unfoldings soon to take place, the other fills the reader with trepidation and tension--two of the staunchest allies of the eternal mystery.
Take one of the earliest scenes in Arthur Conan Doyle's tales of the exploits of Sherlock Holmes, The Hound of the Baskervilles. The lord of the manor, in hosting a dinner party for some friends, took time out to run food and drink down to a young maiden, whom he held captive, when he discovered her cage empty.
Racing back up through the house, he beseeched his guests to join him in pursuit. ".... Whereat Hugo ran from the house, crying to his grooms that they should saddle his mare and unkennel the pack, and giving the hounds a kerchief of the maid's, he swung them to the line, and so off full cry in the moonlight over the moor.
"Now, for some space the revellers stood agape, unable to understand all that had been done in such haste. But anon their bemused wits awoke to the nature of the deed which was like to be done upon the moorlands. Everything was now in an uproar; some calling for their pistols, some for their horses, and some for another flask of wine. But at length some sense came back to their crazed minds, and the whole of them, thirteen in number, took horse and started in pursuit. The moon shone clear above them, and they rode swiftly abreast, taking that course which the maid must needs have taken if she were to reach her own home.
"They had gone a mile or two when they passed one of the night shepherds upon the moorlands, and they cried to him to know if he had seen the hunt. And the man, as the story goes, was so crazed with fear that he could scarce speak, but at last he said that he had indeed seen the unhappy maiden, with the hounds upon her track. 'But I have seen more than that,' said he, 'for Hugo Baskerville passed me upon his black mare, and there ran mute behind him such a hound of hell as God forbid should ever be at my heels.' So the drunken squires cursed the shepherd and rode onward. But soon their skins turned cold, for there came a galloping across the moor, and the black mare, dabbled with white froth, went past with trailing bridle and empty saddle. Then the revellers rode close together, for a great fear was on them, but they still followed over the moor, though each, had he been alone, would have been right glad to have turned his horse's head. Riding slowly in this fashion they came at last upon the hounds. These, through known for their valour and their breed, were whimpering in a cluster at the head of a deep dip or goyal, as we call it, upon the moor, some slinking away and some, with starting hackles and staring eyes, gazing down the narrow valley before them.
"The company had come to a halt, more sober men, as you may guess, than when they started. The most of them would by no means advance, but three of them, the boldest, or it may be the most drunken, rode forward down the goyal. Now, it opened into a broad space in which stood two of those great stones, still to be seen there, which were set by certain forgotten peoples in the days of old. The moon was shining bright upon the clearing, and there in the centre lay the unhappy maid where she had fallen, dead of fear and of fatigue. But it was not the sight of her body, nor yet was it that of the body of Hugo Baskerville lying near her, which raised the hair upon the heads of these three dare-devil roysterers, but it was that, standing over Hugo, and plucking at his throat, there stood a foul thing, a great, black beast, shaped like a hound, yet larger than any hound that ever mortal eye has rested upon. And even as they looked the thing tore the throat out of Hugo Baskerville, on which, as it turned its blazing eyes and dripping jaws upon them, the three shrieked with fear and rode for dear life, still screaming, across the moor. One, it is said, died that very night of what he had seen, and the other twain were but broken men for the rest of their days ...."
Just a light-hearted ditty? Hardly. Read at any time of day or night, the scene inspires chills and even fear in the reader--this, in part, for the remarkable clearness with which the author detailed the scene, using words that still today--nearly 200 years later--bring forth mental images of such clarity and reality that hardly anyone could walk away untouched.
Doyle described: the moon shining bright upon the clearing. The unhappy maid where she had fallen, dead of fear and of fatigue. The thing plucking at [Hugo's] throat ... a foul thing, a great, black beast, shaped like a hound. When suddenly the thing turned its blazing eyes and dripping jaws upon them.
As in the best descriptive passages in literature, Doyle used imagery as a painter uses oils on canvas. A stroke of his literary brush evoked not only a mere understanding of the artist's words, but also a feeling, a string of emotional responses, that a more careless or lazy writer might never have known.
Unlike descriptions of people or things, however, Doyle's descriptive passage was designed to set the scene and--in so doing--set the stage for the rest of the novel. Something evil lurked among the moors of Baskerville manor. Something frightening and unearthly, some devil of a creature unlike any other known to man. Something hideous. Something immortal. Something ... well, you get the point. By the time the passage had ended, the stage was set for mystery.
So the next time you set out to pen a mystery--whether a novel or a short feature--remember to pay special attention to the scene. That is, after all, where everything within the story lives, from characters to action, from story line to story's end. Set the scene properly, evocatively, and early, and you'll find that the rest of the story will practically write itself. Oh, yes, and your readers will thank you for it.