First of all, you need to understand that scripted TV cop/detective dramas are generally divided into three acts. The detectives usually have somebody they're focusing on in Act One, and another chief suspect in Act Two. Neither of these suspects will be the murderer. To be fair, I should say that there is a very occasional plot twist in which the Act One suspect––who will have been cleared of suspicion either during Act Two or by the end of Act One––will suddenly be revealed as the murderer in Act Three. This is like the Statue of Liberty play in football, though; meaning that this plot twist is only effective if used extremely rarely and judiciously.
|The Prisoner's Patrick McGoohan played the killer four times on Columbo|
Next whodunnit rule: If one of the suspects is a super-rich captain of industry, that's your man––unless it's one of his kids or close associates. Assuming that there is no zillionaire character and there isn't a well-known guest star in the episode––or if there is but the guest star seems to have a lot of face time––then the general rule is that the murderer will turn out to be a character who is introduced in Act One but who is not a suspect. For example, this will usually be a witness interviewed by the detectives; somebody like a friend, co-worker or schoolmate of the victim, a relative of the victim's spouse. If you're watching a show, pay attention to these witness characters and amaze your friends by predicting the whodunnit before the end of Act One.
Now that we've covered the basics of the whodunnit, we can talk about a few other conventions of crime drama. These don't lend themselves quite so well to impressing your friends, but you can still use them to make wise-guy remarks––and that's always a vital part of the TV-watching experience.
Our next convention concerns crime-detection technology. The police on TV have some amazing technology. Computerized whiteboards, huge touch screens with multiple displays that allow them to graphically represent all of the elements of the case, mind-boggling medical forensics tools and, my favorite, facial recognition software that instantly provides the ID of somebody caught on some grainy surveillance camera from a database of, apparently, everybody in the entire country. Half the technology on these shows doesn't even exist in real life, let alone in cash-strapped cop shops. The only vaguely realistic part of the whole cop/technology thing is that federal crime agencies seem to have much fancier technology than city police detectives.
It almost goes without saying that on TV crime drama, all criminal defense lawyers are jerks who treat cops with utter contempt. I don't even know why I bothered writing about this convention, since everybody already knows this one.