There are some things you want to be invisible, like panty lines, pet hair (that's taken up residence on your shirt), and pimples. And there are other things you definitely don't want invisible, like doors, fast-moving cars, and your pants. One of the jobs of a writer is to successfully sort things into those two camps, and assign words accordingly. Otherwise, you end up with plenty of panty lines, pet hair, and pimples, but no pants, as you slam into an invisible door, fall, and are painfully but not fatally run over by . . . something. Look, it had tires, if the tracks on your shirt are any indication, but after that, you really have no idea.
Anyway. That was a really long metaphor to tell you something that's actually pretty simple: choose the right words. Simple right? Not exactly. Hence the metaphor. There are three basic ways words can sin in the world of writing, thus earning their execution at the hands of the almighty editor. They can be unnecessary, they can be blah, or they can be just plain wrong (for the occasion, anyway).
We tend to develop an instinct for this as we read, but it can be super helpful to break it down sometimes, especially when editing one's own work, or when trying to figure out why a sentence just doesn't have that special oomph you wanted. For your convenience, I've broken out the three types of bad language I mentioned above, why you should cut them out of your prose, and when you can actually use them to better your book.
fell to the ground
nodded his head
blinked his eyes
Unnecessary words are just that: unnecessary. Meaning, the reader gets just fine what you're trying to say without them, so all they do is slow the prose down. For instance, when you use a word like "ran," the default is "forward," so unless your main character's running backward or sideways or in some other entertaining direction, you can leave the direction out. Likewise, when you "fall," we assume it is "to the ground," unless you tell us otherwise. And what else would he "nod" but his "head"? Or "blink" but his "eyes"? Cut, cut, cut, and cut.
When to Use Them: In some cases, unnecessary words become necessary because of other information that comes to light. For instance, "he ran forward, then backward, then sideways" makes sense where "he ran, then backward, then sideways" does not. And sometimes, it helps the rhythm of the sentence: "he fell to the ground, bounced once, then lay still," has a better rhythm (and paints a clearer picture) than "he fell, bounced once, then lay still." Also, there's a whole class of unnecessary words that are fantastic for evoking a conversational tone, or for keeping one's prose from assuming a sausagelike density. I use those kinds of unnecessary words a whole lot.
He moved across the room
Emotions and thoughts ran through his head
He continued on
Blah words are forgettable. Bland. Empty. They're placeholders, underlines, and reminders to go back and put in a more interesting word"”one with a little character. And they're premo zero-draft material! But when you get to your final draft? Blah words have no place in it"”at least no place where they have to carry any weight. You want your words to paint a picture. And words like "moved" give me absolutely no image. No sense of character. No idea as to what the character's thinking or feeling. Just a character name slid across a mental grid.
In good books, words are specific and evoke specific imagery. Different characters would execute the same move differently"”just as their motivations and personalities are different. And likewise, the same character would execute the same move differently depending on his mood. Saying "he moved across the room" robs us of that extra bit of personality as well as the specificity that brings a book to life. Anyone (or thing!) can move across a room. But how would a proud warrior "move"? Would he stride? Or march? Is he sauntering, swaggering, and occasionally catching his balance on chairs, having had a little too much to drink to behave as a warrior of his station should?
When to Use Them: You've heard "never use a complex word when a simple one will do"? That's where this comes in. When all your characters skitter, skate, creep, scuttle, skip, dash, race, sprint, and tiptoe everywhere they go, it's going to sound a bit like you ate the thesaurus. There's no point in making everyone careen about out of character just to shunt some more verbal diversity into your manuscript. Especially not when you can augment simple words with delightful similes or metaphors or other evocative combinations of words to bring the imagery. In fact, in those cases, it's often best to use a simple word so that the sentence doesn't get too purple. Likewise dialogue doesn't often benefit from fancy tags as the weight is on the words being said, not those describing the saying. (I am, for the record, a huge fan of "said.") Just make sure that when you use a blah word when writing description, you've got something else in there to carry the weight of the imagery.
The Wrong Words
Not Sexy: He moved his hand up and down her right forearm four times in succession, agitating the skin in order to raise the temperature of her flesh"”and her heart.
The wrong words are fine words"”but they are just terminally ill-equipped to deal with the situation to which they've been assigned. It can be anything from using incredibly technical language to describe a pastoral landscape to plopping distant 3rd person lingo into tight 3rd person paragraphs. At best, it's slightly awkward. At worst, it's almost unreadable. This is also sometimes the reason otherwise brilliant fight scenes feel off: that collection of super dry, multisyllabic words you've been saving up just doesn't belong in it.
Even if a word is right on target, you need to consider the surrounding words before plopping it in there. Rare and unique words, beautiful though they may be, are generally one-a-paragraph affairs: divas that they are, such words demand a lot of attention, and require the proper words around them to show them to their best effect.
When to Use Them: Mixing up terminology with a situation it's not usually associated with is great for humor. It's one of my favorite kinds, in fact. Using incredibly technical terms to talk about someone's level of inebriation at a party. Using awesomely baroque terms to discuss a hip hop video. But play with it to develop an ear for it before you use it for real"”and read examples of people doing it well, because there's humorous off, and then there's just off.
Specific, Simple, Germane
Of course, with 90-some thousand words in any given novel, it's not like you really have time to sit down and have a serious think over each and every one. But in general, if you choose words that are specific, simple, and germane, you'll come out ahead in the end. And never let a diva of a word disrupt your flow"”there will, after all, be other sentences.