Sir Arthur Conan Doyle's bestselling "Sherlock Holmes" mysteries set the archetype for detective and sidekick characters. Holmes, a cognitive genius, is always one step ahead of his assistant, Watson. Some detective writers eschew the sidekick character because it has been overdone, but classic detective fiction includes a sidekick. The detective should also stay one step ahead of the reader. Along that line, detectives should not be hopeless in their craft, unless the author is aiming for comedy.
A good crime story involves a murder. Detective fiction with only lesser crimes is simply not as intriguing. Murder stories necessitate characters with desperate motivations who weave layers of deception to cover their trail. This creates the perfect situation for a sharp-witted detective to unravel but keeps them on their toes and bumping against dead ends from time to time.
Smart detectives take stock of all possible suspects in a case, assuming the innocence of no one. If the murderer is too obvious, however, and the detective suspects him right from the start, the climax of the story will hold little surprise for the reader and the novel overall will disappoint.
Effective detective fiction writers know what kind of guns murderers are likely to use, the details of obtaining search warrants and the effects of certain poisons. Inventing scientific developments or alien species to account for the success of a crime comes off as forced and unbelievable.
Every detective fiction reader eagerly waits for the final showdown between the detective and the criminal in the climax of the novel. In this scene, the clever detective has already put the evidence together to figure out the criminal's motive and method, but she still has to confront the criminal so the reader can hear the murderer's confession for himself.