From The Writers Digest, April 1931:
Writing a mystery story is like playing a game of chess with a thousand unknown opponents.
As a matter of fact, the game is one more fascinating than chess, and more intricate, for in it the pawns and bishops and knights are replaced by human characters whose value as pieces varies as widely as the poles, and because the “moves” are twists of plot and situation which are not limited by neat squares on a board. But definite rules do exist, and if reader or writer does not conform to them strictly, his opponent may justly raise the cry “unfair!”
The regulations governing the reader’s part in the game are simpler than those controlling the writer. He can only cheat in one well-recognized way, by looking at the end of the story or novel before he should. The author-player, on the other hand, is bound by a number of restrictions. If he disregards any one of them, he has not played square.
But the writer, once he has these rules clearly in mind, can have no more entertaining diversion than this, of pitting his skill against his readers. In spite of the greater complexity of his rules, or perhaps because of it and because of the mentality of his reader-antagonist is an unknown quantity to him, his is the more exciting side to be one. He, after all, is the chief player; the reader must follow along as the author chooses to have him. And so, within the rigid formula, the writer of a mystery story is referee and umpire as well as player.
The rules controlling the writer are not as complex as they might seem at first. Everyone who has read detective stories has spotted various unfair tactics on the author’s part, and is familiar with certain of them.
From such criticism it is easy to collect a fairly obvious list of “do’s and don’ts” for the writer, a sort of Hoyle for the constructor of mystery fiction.
In the first place, the author must not introduce some character at the last minute, a deus ex machina, to be revealed as the murderer or thief. The guilty man must have put in an appearance early in the story, and be well known to the reader throughout. How he may be introduced, and still be covered up from the reader’s suspicion will be touched upon later. This is the chief rule, and the most evident one.
In the second, the writer must not deliberately inject inconsistencies with the narrative to blind the reader to the identity of his guilty person. This is equally evident, and should not be taken to mean that the murderer cannot have an apparent “cast-iron alibi” or apparently no possible motive for the crime. Only facts which cannot be, or never are, satisfactorily explained are barred.
. . . .
It is common experience, I believe, and I have found it to be true, that it is safer to work from the “checkmate” backwards, so to speak. The writer first devises the method of killing or an unusual motive; then, with this in mind, develops his characters and plot to work up to a revelation of this first idea.
The simplest way to do this is to ask, “If I were going to murder So-and-so, so as to escape being hanged by the neck until dead, how would I go about it?” Fortunately, all of us know people with whom we could do away with pleasure, and this adds zest to the work! The next bit of self-interrogation is, “Why do I really want to commit homicide on So-and-so?” With a strong intensification of the answer to this question, or a slight modification downwards of his own character, the author has his motive.
For example, with all my heart I would like to kill Editor Jones, I know his working hours, approximately nine to five. I know where his office is located, up how many stories he takes an elevator to work, and by what street-car line he travels between desk and home. But I do not wish to go to the Chair after killing him. Life though good, would be much better were Editor Jones not in it. Now, shooting this individual would be gratifying, but firearms make a loud noise and could be used only on a dark winter afternoon as he walks from the trolley to his house. I therefore note those conditions as the best under which to shoot the abhorred publicist. Perhaps stabbing editor Jones would be nice, and certainly it would attract less unwelcome attention. Where could that be done, and how? Perhaps a painful poison would be most amusing of all. Where may it be best administered? And so on.
Then, why do I wish to remove this gentleman? Because he rejected that five-page narrative poem of mine called The Charge of the Violets, and was sufficiently rude about it, too. But changing myself into a Bolshevik, who has had an anti-bourgeois paper turned down, I may have an acceptable grounds for murder.
Link to the rest at The Writers Digest