A shaking hand draws a crude letter A in the spreading pool of blood...
The detective notices something flash in the light and he reaches down to pick up a brooch...
A witness coughs whenever the time of the scream is mentioned...
Welcome to the clue, a fixture in most mystery writing. Poorly executed, clues will keep an author in the slush pile forever. Done well, clues are the answer to everything.
Clues serve two distinct functions. They lead the reader to the solution and they mislead the reader away from the solution. They've very versatile.
For the sake of discussion I'm going to break clues into three types: physical, verbal, and thematic.
While the first two types of clues will influence the detective, all three will affect the reader's experience of the story.
Clues enlighten readers and clues confuse them.
Time for another division of the general clue: immediate clues and future clues.
A single shoe is misplaced in the closet. This is an immediate clue which may or may not be noticed by both the detective and the reader when the scene is described.
An autopsy is ordered. The reader cannot learn the results until the detective does.
There is a trade-off. Immediate clues allow the reader to match wits with the detective. Future clues build suspense.
When planting clues, there are a number of tricks you can use.
As if there aren't enough ways to split the clue, here comes another. There are obvious clues and subtle clues.
The button clutched in the dead hand is an obvious clue. No one is going to miss the importance of the object.
The fact that the victim was wearing blue is a subtle one. Perhaps it has a meaning and perhaps it doesn't. Some readers will file away the information and others won't.
All of these clue types can and should be mixed and matched. The complexity of direction and misdirection is limited only by an author's imagination and the length of the manuscript. Whatever level of convolution you can dream up, there is a group of readers looking for that experience.
Like a magician, writers of mysteries must mislead the audience while fulfilling expectations of entertainment and surprise. There is one large difference between the two arts, however. Readers want to end the show knowing they could have solved the trick if only they had paid more attention to [enter major clue here]. It is the writer's job to make sure they don't.
Magicians on a stage work within a closed environment. Audiences see the props and magicians "prove" that there are no hidden devices. A story is not a closed environment. The writer can theoretically bring anything onto the stage at a moment's notice just by writing the words. "Here comes the stealth helicopter lowering a basket. Look, I just allowed my hero to disappear."
Because this is possible, it isn't really allowed. Devices and clues should appear and be visible before they play their part. If you want a stealth helicopter to arrive and rescue your main character, you want to plant the possibility of that occurring earlier in the story. If the killer is arrested because of a fingerprint lifted from a shell casing, you want to mention the discovery of a shell casing if not describe the actual lifting of the print. If a shotgun is fired in the third act, you want to show the gun hanging over the fireplace in the first. (Chekhov reminds us that if you place a shotgun in the first act, you must have it fired by the third.) The reader should see the wires and mirrors but not recognize their significance. This is how the writer plays fair.
Magicians banter with the audience and mumble incantations. This buys them enough time to slip the key out of the hidden pocket but more importantly, it focuses audience attention. Misdirection is also the mystery writer's best friend.
Let's say you need that stealth helicopter to rescue your hero and you don't want the reader to be disgusted when it drops in with no advance warning. How do you mention the helicopter earlier and still let the reader sweat the character's situation?
You could have your character talking to some person who keeps looking out the window because he's watching for a delivery truck. "I'm waiting for my new rotor blade." You could have your character discussing the helicopter with the pilot and being disappointed with the pilot's personal use of company property. The focus is not on the helicopter but a question of ethics. You could have your character talking to a pilot who offers -- begs -- to be included in the wild adventure of an investigation. The emphasis is placed on motivation. In this case, the stealth helicopter can come as a surprise because the reader believes the pilot will do anything to become involved.
Now, none of these efforts are particularly stealthy, but few of the things you'll be trying to hide will be as large and unique as a stealth helicopter.
Magicians usually stick to a common set of props because we are familiar with the objects and their properties. The magician who pulls a ferret out of his hat instead of a rabbit has toyed with audience expectations. The gamble may or may not pay off.
"I'm waiting for my new rotor blade." If that is your sole clue, you're assuming readers will know that "rotor blade" implies helicopter. You're also assuming that "rotor blade" doesn't mean anything else to readers with a different world view. Using subtle clues can be risky, especially if the knowledge isn't considered mainstream. If the average reader can not be expected to understand the clue (even after re-reading), you should use a direct/misdirect approach.
"Hey, guess what? I've been promoted and now I get to fly the stealth helicopter."
"I wish my news was as good. I'm sorry, but your sister is dead."
You've openly given the reader information that will quickly be forgotten due to the emotional nature of the unfolding scene.
The opposite of the emotional direct/misdirect is the informative one.
"So my boss hands me this folder with all the items I'm responsible for and tells me I need to tag them with inventory stickers. I start thumbing through the sheets. One metal desk, beige. One motorcycle. One filing cabinet. One laptop. One helicopter, stealth. One stapler. Fifteen desk chairs. Fifteen chairs? Why would I have fifteen chairs?"
The danger of hiding a clue within a list is that readers may recognize a list is coming and skip ahead. While you can stand on high ground and berate readers for not savoring each and every word, this is not the way to build a fan base.
The magician signals the end of the trick with a flourish. The people in the audience don't know how the trick was achieved but didn't expect to. They whistle and applaud.
Mystery readers expect the trick to be exposed. They also expect to feel that they should have seen it coming, that the clues were right there in the open, and that the puzzle was fair. A writer cannot gauge an audience's reaction and try to win them back. You can't defend your choices and change the reader's mind. If you lose your readers, they're gone. Which brings us to...
People read mysteries for many reasons. One of the most common is the chance to match wits with a detective, to solve the crime or at least nod with appreciation when the answer is unveiled.
Clues are the way you dance with readers, alternately pulling them closer to the truth and spinning them out to the dizzy edge. Keep readers on their toes, keep them guessing. They'll love you for it.