An easy rule in good writing (or, specifically, rewriting) is this:
Steadfastly avoid using the word “very.”
Furthermore, when you go to cut it out, try to resist replacing it with a more specific adjective (“vastly,” “immensely,” “extraordinarily”). Any of those is better than “very,” but they don’t dodge the problem. It’s much, much better to say “She moved at a jog” or “She moved at a sprint” (see how much control over the visualization that gives me?) than to say “She moved at a very fast pace.”
There is a perfectly explicable reason behind the rule. When you are narrating a story to someone, your goal is to make them believe it. When you say, “The castle was big,” as the narrator, your goal is to make the reader think of a big castle. And, helpfully, the reader’s fist instinct is to believe you, and imagine a big castle. You can say it was “enormous” or “monstrous” or “ponderously large” and all of those evoke slightly different variations on the mental image. However, when you say, “The castle was very big,” it doesn’t actually make the image any bigger. “Big” already told the reader what to imagine. When you say “very big,” the reader’s first reaction is “How big?” And, immediately on the heels of that comes the thought, “Wait, how big was it really?“
In other words, the effect of adding “very” to any description in text actually serves to make the reader question the authenticity of the narrative. Your goal as a writer is to make the reader believe you (except in rare and artsy-fartsy circumstances), and by instinct the reader does, right up until you toss in “very.”
You can get into even more trouble using words designed solely to convey authenticity, throwing in “honestly” and “truly.” It’s natural for writers to do this — it’s exactly how someone would convey intensity when telling a story around a campfire — but there is a vast and inescapable difference between a campfire story and a novel.
It’s not just a matter of style or voice, either. It’s a matter of psychology. When your audience is reading a story, they process the information provided to them in a fundamentally different manner than they use when someone is telling them a story in person. Good speakers often have the same problem as good storytellers when they try to write down a story, because the rhetorical tricks that people use in speech, even employed flawlessly in book, just don’t work the same way in print. In fact, effect is often the opposite of what you intended.
Because, just like with “very,” when your narrator says, “honestly she was relieved that he had fainted,” the reader’s immediate reaction is to think, “Wait, how honest is that really?” And that’s the opposite of the effect you want. That’s the reader questioning your narrator, which means for a while he won’t believe anything the narrator tells him.
Whenever I give rules on the construction of sentences (and, just, generally the way you say things in a book), there is a major exception in place for dialog. In dialog, your only goal is to realistically represent the way people sound when talking. You can do characterization by having a character use weak verb forms. You can cast doubt on the credibility of a character by having him say “honestly” and “truly” every other sentence. You can use mixed metaphors and sentence fragments all you want in dialog, because it’s supposed to sound how that person sounds, and most of our bad writing habits come from perfectly acceptable speech.
So you don’t have to go through your book and delete every single “very,” because some of them will be in dialog, and people say “very” all the time in speech. Actually, that’s an important point. The reason we don’t bother teaching people (other than writers) to avoid “very” is because you don’t have the time to pick the precisely accurate noun and verb for every sentence when you’re rattling off ideas one after the other in a conversation. That’s also why I clarified at the top of this post that avoiding “very” is a rule for rewriting. It is perfectly fine to use in a rough draft narrative, but needs to be cleaned out in the rewrite, when you do have time now to pick the right words.