There are a few things — more than a few, actually, but this is a blog post, not an exhaustive list — that you’re going to get the wrong impression of, if you’re relying on film and television and not real life:
How dark darkness really is. Scenes on television and in the movies that are supposedly set in lightless or minimally-lit places (the woods on a moonless night; a windowless room) are in fact taking place in a representation of darkness and not the real thing, and the representation has to have enough light going on that the viewers can follow the action. You’re a writer, not a film or television director, so you don’t have access to that particular artistic convention. You need to keep track of what your light sources are, and if you don’t want your characters to be tripping over furniture in the dark, have them remember to bring along a flashlight.
How much injury it actually takes to put somebody out of action. If all you want to do is sideline a character for a few chapters so that, for example, other characters are temporarily deprived of their assistance, it’s not necessary to riddle them with bullets or put them in a coma. A severe sprain, a minor dislocation, a bad case of flu or even food poisoning . . . any of those will work as well.
How loud gunshots really are. Make that how LOUD. Your characters aren’t going to be holding any complex conversations in the immediate aftermath.
At what speed the wheels of justice really turn. Anybody who’s ever served on jury duty knows that the reality is a long way from its fictional counterpart. There are fewer moments of high drama, and more moments that sound like a couple of highly-paid professional litigators playing a complex but boring game of Mother-may-I. (If you’ve never served on a jury, do so if you’re called — the experience, for a writer, is invaluable. Lacking that opportunity, you can sometimes find gavel-to-gavel trial coverage on television or the internet.)
The moral of the story, unsurprisingly, is that if you find yourself writing about something that you only know about through media representations . . . back off and do some research.