There are some tropes that anyone who enjoys weekly detective stories will be familiar with. That’s not to say that these are necessarily bad, but these are things you should probably be aware of when you are watching.
The most sigificant thing is that the investigative timeline is usually massively compressed. This is particularly the case for the current crop of police procedurals. It is common to see the entire investigation, from the discovery of the crime to the final arrest of the perpetrator, pass in the space of at most a handful of days. While it’s true that sometimes an investigation can wrap up that quickly, it is a bit unrealistic. For one thing, the police and other people involved in the investigation are busy people and likely have other investigations ongoing. For another, many of the “scientific” tests that get done take some time to complete. This trope is typical used to cut out the boring parts of an investigation but it also leaves an unreasonable impression of how long an investigation actually takes.
Another thing that shows up a lot is that the detective will be working on exactly one case. This might be reasonable in a small town setting where there isn’t a lot of crime that needs investigating. However, that same fact also means that the small town is unlikely to be a suitable place for a weekly series without falling prey to Cabot Cove syndrome. Some setups do allow for this to make sense, though. For instance, a “special crime” task force or something that gets to choose its cases such as in The Listener.
Another thing that has become common is what I’m calling Magic Forensics. This ranges from the infinite zoom and enhance thing we often see with images to more plausible things like infallible ballistics and gun shot residue tests. And that’s leaving aside magical DNA tests on nanoscopic residue and other such things. Sometimes this is done to shortcut a more plausible but mind numbingly boring process. However, often this sort of thing is done just because the viewers expect it or because it looks cool. The problem here is that a more realistic approach to forensics would lead to potentially better stories and wouldn’t leave the viewers thinking they know something when, in fact, what they’ve just seen is nonsense.
Finally, one of the things that makes the whole police procedural genre iffy is that the same people are often shown investigating the crime scene, collecting forensic evidence, processing said evidence, forming theories, and even arresting the perpetrators. This is usually done to avoid having to pay a ridculous number of cast members to show up for only a minute or two in each episode. However, in actual reality, scientific testing is usually done by specialized labs, evidence is collected by field technicians, and the case is handled by one or more detectives or similar who manage everything and direct resources. That’s an oversimplification, but it gets the point across. The reason it isn’t all shown is a practical one from the production perspective. That’s not to say it can’t be done accurately, though. Murder She Wrote usually gets this right, for instance (they simply mention “the lab boys” and the like, and look at reports).
There are others which confuse matters, too. For instance, eye witness accounts almost always match exactly what we saw on screen when, in fact, witnesses are not reliable at all, except when a deliberate Rashomon plot is being developed.
Next time you’re watching a crime drama of some sort, or even reading one, pay attention and look for these types of things. That can be as interesting as the actual story. Sometimes it can be more interesting.