Detective Fiction: basic guidelines
This is what I posted on the Facebook group The Golden Age of Detective Fiction, which I created and made myself Grand Panjandram of. I am, at the moment, the sole member, but if you agree with what follows, you could always join. It will be a very benign dictatorship. Until you disagree with me. Then things will get ugly.
Some general principles:
The classic detective story is a battle of wits between criminal and detective in which the criminal or at least the means of his having committed the crime remain unknown until he (or it) is unmasked by the detective by means of evidence which has been fairly presented to the reader.
No such evidence should be concealed from the reader. In a good detective story it is positively dangled before the reader (the missing item in The Five Red Herrings, by Sayers, or the corpse's clothes being on back to front in The Chinese Orange Mystery, by Ellery Queen, are good examples).
The plot should be soundly constructed, without loose ends. We care who killed the chauffeur, even if Chandler doesn't.
Al the characters should act according to their circumstances and plausible motives: if everyone at the country house party is waltzing through the library where the baronet's body lies with the Egyptian paperknife in his neck as if it were Grand Central Station, each of them shedding physical evidence on all sides, they should at least have a reason to be there.
The investigation of the crime (fingerprints, physical evidence, witness statements) is of no intrinsic interest except in presenting the puzzle. There is nothing wrong with the police procedural, but it is not a detective story.
The crime should not be solved by killing everyone else until the killer is the only one left. There is nothing wrong with the thriller or the hardboiled private eye novel, but it is not a detective story.
The killer should not be the detective, or (except exceptionally) a servant, or anyone whose thoughts we have been privy to.
The crime should ideally be the work of one person. Confederates are tolerable, but unsatisfactory. Murder on the Orient Express (Christie) is a swizz. Other characters may confound the crime, however, as in Death at the President's Lodging (Innes), so long as they act with motives which are comprehensible, and don't just shamble about messing things up for no reason.
The revelation of the murderer's identity, or his method, should ideally produce in the reader the response "Of course!" It should come as a surprise, but not a further mystification.
The best conclusion of a detective story is one which produces an effect similar to that of a cryptic crossword clue which has had you stumped for ages, and which you finally see. Wit and ingenuity are the most satisfactory results.
Plausibility and realism are important only within the terms of the novel's world. It is not plausible that people should be murdered in hermetically sealed rooms, but the solution must be workable, not necessarily very likely.
Any one of these guidelines may be broken if it is broken with sufficient elan.
