All throughout school, we are taught to use proper grammar and effective style. We must write and communicate well in order to be taken seriously. A poorly written essay, letter, or advertisement shoots holes in our credibility.
However, fiction writers have a secondary plain on which we operate. Our language must match our story. For example, I was recently reading a fantasy in the Middle Earth vein. The beginning of the book especially drew me in. The world was well-crafted and complete, with its own language and customs. The omniscient narrator had a story-teller feel; I felt as though I were sitting around an ancient fire listening to the recitation of an epic.
As the story moved on, the language became sloppy. I was especially taken out of the story when a character said, “But things can change fast.” In a contemporary novel, this would have been completely appropriate. Coming from a character who always spoke most properly, however, the statement broke the layer of reality the author had so carefully constructed. Things do not change fast, adjective; they change quickly, adverb.
(For those of you who are like me and usually can’t remember the parts of speech to save our lives: an adjective describes a noun; an adverb describes a verb. Change is a verb, thus quickly is the appropriate word choice.)
Alternatively, proper English is not always appropriate for the story. I was reading through my draft, told in close third person from the POV of a teenage boy. The scene is one of intense fear. Flames and battles are raging everywhere and the MC is trying to find his friends to make sure they are safe. I came across this phrase: “In which dorm was she staying?” Okay, how many teenage boys, especially in the face of unknown danger, would say “in which”? I revised: “Which dorm was she staying in?” Proper English can take the reader out of the story as much as improper English can.
Moral of the story: you should use the style of language appropriate for the story you are telling.
Croggon, Alison. The Naming. Candlewick Press, Cambridge, Mass. First US Edition, 2005. Page 240.