Whenever I'm out with a certain friend of mine, he always comes up with a bad idea and he says "I saw it work in a movie once! Let's try it!". I sort of want to hit him every time he says that.
But on the other hand, I do indeed love Law and Order.
Well, Law & Order portrays legal situations fairly accurately. The cops can lie to you, they will try to get you to roll over on your friends (often by telling you they already confessed), and they can't force you to answer any questions.
That being said, although a non-response can't be used against you in court, detectives will take a non-response as evidence that they're on the right trail. "Why did you have the shovel in your trunk?" "I refuse to answer" -- they'll then try and find a reason.
At any point, you can also say, "I'm not answering any more questions" or "I want a lawyer" and they have to respect that.
I've been watching Law and Order for the whole long weekend, so I know all the tricks! But we also learned this back in school.