The best detectives are divorced alcoholics with gambling addictions who cut corners wherever they can, have problems dealing with authority figures, and haven’t gotten over the death of their child several years ago.
If the opening scene includes a background shot of a billboard promoting a political candidate, the political candidate turns out to be the bad guy.
When someone unexpectedly knocks on his door and the protagonist takes out his gun and cautiously turns the door handle, he never ends up needing the gun.
If you don’t see the body, the character isn’t really dead.
Criminals don’t know any medical doctors. That’s why when they’re on the lam and badly wounded, they always get patched up late at night by a veterinarian.
The fancy knife holder on the kitchen counter is the film equivalent of Chekhov’s Gun.
When they can’t find a shady veterinarian, the criminal or wrongly accused protagonist has the knowledge and skill to perform the necessary surgery on himself.
Protagonists who are professional hit men are cultured connoisseurs of either fine art or fine wine. They have strict moral codes, impeccable manners and a dozen different passports.
In the climactic shootout, the Big Boss of the bad guys always dies last.
No good ever comes of a woman walking toward her car in a deserted parking garage.