That and which. There's a difference. A lot of people don't know that. ALOT. (That's a joke. Look carefully.)
Here's the basics - if the clause introduced by that or which can be taken out of the sentence without affecting its central meaning, use a "which" with a comma in front of it. If the clause is required, then use a "that" with NO comma. Some examples:
The cars, which are on the lawn, are broken. - this means that the cars that are broken happen to be on the lawn. "Which" indicates a non-restrictive clause; a clause that provides more information about the noun in question, but that information is not essential. It's a "by the way" statement.
The cars that are on the lawn are broken. - this means that the cars that are on the lawn are broken. "That" indicates a restrictive clause; a clause that provides essential information about the noun in question.
Is this a big problem in fiction? No, not really, because oftentimes, in fiction, either meaning is okay. People sometimes misuse "which" as a "formal" form of "that," so sometimes, I'll see a "which" in a book without a comma in front of it, but whatever. It usually doesn't change the meaning of the sentence in the context of the story, so no big.
But in my day job as an attorney, it is a big problem. Because restrictive clauses are requirements and non-restrictive clauses may not be. This is how the error usually manifests itself:
"The merchandise which shall be shipped on the 15th is guaranteed to be free from defect."
Okay, is that clause restrictive or non-restrictive? There's a "which", but there's no comma. So if I read that in a contract, I will change it to the term that's the most favorable to my client. If I want the clause to be a requirement, I will change it to "that." If I want it to be less forceful, I'll add a couple of commas. Most of the time, the lawyer on the other side won't understand that there's a difference, even after I explain it to him or her. I'll just get a "whatever, fine," even after I'd made something mandatory that wasn't mandatory before.
Sometimes, I feel like knowing the rules of grammar is the legal equivalent of reading the rules in the top of the Monopoly box.