I recently read SIRE AND DAMN, the latest (I hope not the last, but I fear it may be) "Dog Lover's Mystery" by Susan Conant. It started me thinking about the conventional but "unrealistic" elements mystery authors have an implicit agreement with readers to treat as believable, similar to the theatrical convention that actors can speak "aside" to the audience without being heard by any other characters onstage.
Most obvious is the convention that makes amateur detective series possible at all. We have to accept as normal that a hero or heroine not involved in law enforcement, an investigative profession, or the criminal underworld encounters dozens of murders in his or her daily life. It's the phenomenon that gives the small Maine town in the TV show MURDER SHE WROTE a higher per-capita homicide rate than Baltimore or Washington. Sometimes the author offers a sort-of rationale for the protagonist's repeated clashes with violent death. Walter Mosley's series protagonist Easy Rawlins, a black man in post-World-War-II southern California, builds a reputation in his community for solving problems people don't trust the police to deal with. Similarly, Barbara Hambly's "free man of color" in antebellum New Orleans, Benjamin January, after unraveling a few mysteries in which he accidentally gets entangled, becomes the person his friends and acquaintances—black, white, and mixed-race—turn to when delicate problems arise. Most "cozy" mystery series, though, simply ask the reader to accept the premise that a chef, a writer, or (as in Susan Conant's series) a dog trainer will trip over a murder or two every few months.
Likewise, we expect the murderer to be revealed as a member of the heroine's circle of acquaintances, not someone from out in left field she's never met. In the classic tradition established by such authors as Agatha Christie and Dorothy Sayers, pinning the crime on a character not included in the roster of suspects would be considered unfair to the reader.
The more hard-boiled varieties of murder mysteries, the kind with a higher level of onstage violence, feature another "unrealistic" convention: The hero or heroine usually gets knocked unconscious at least once per book. Yet he or she recovers (sometimes after a credible period of recuperation, sometimes "unrealistically" fast) and soldiers on through adventure after adventure with no sign of permanent brain damage. Given all the recent media publicity about the dangers of concussions (in children's athletic programs, for instance), we have to accept the hero's phenomenal toughness and good luck as part of our genre expectations.
More often than chance would predict, early in the story the protagonist comes across just the bit of specialized or confidential information that she'll later need to solve the case. This example seems to me a borderline case; the effectiveness of suspension of disbelief depends on the author's skill in planting the information in the natural flow of the action. It stretches the bounds of credibility, however, when the amateur detective just happens to overhear a fragment of dialogue that conveys the vital piece of missing information. There was a TV series about a crime-solving priest and nun that, although it was lots of fun, did that kind of thing too often. One of Elizabeth Peters's suspense novels satirizes this device when the heroine sneaks into the villains' lair and eavesdrops on them, lamenting that nobody conveniently says something like, "I will now go to the dungeon and check on our prisoner, who is in the third cell on the right."
In general, any reliance on coincidence to solve the mystery is problematic; an author might be allowed one such incident every now and then, but a little bit of coincidence goes a long way. There can be a fine line between disbelief being suspended and (as Marion Zimmer Bradley used to say) hanged by the neck until dead.
Then there's the direct opposite, Tolkien's "secondary belief," when an author draws the reader so deeply into a fictional world that it comes to life for us. No "suspension" is needed, because we experience the secondary world as a realm that could truly exist on a different plane of reality.
Margaret L. CarterCarter's Crypt