Nothing scotches believability faster than a character who doesn’t sound right. Much of the time these are mistakes of vocabulary, like a third-grader who uses the word “disenfranchised” or a hardened felon who says “Darn tootin’!” But just as important, or even more so, is the question of metaphor.
(As I write about this, I’m going to use “metaphor” as a category that includes both metaphors and similes. The difference between them is a question of grammar, not concept. And although I am without question a grammar stickler, I think dropping the distinction makes sense for exploring their use. Otherwise things get all cluttered up, saying “metaphor/simile” every time, and it gets as messy as a roomful of toddlers who’ve skipped their naps.)
A lot of writers probably come to this realization automatically, but I didn’t. I had to learn it. One summer at the Iowa Summer Writing Festival I went to an Elevenses lecture by Sands Hall. I don’t remember if this was the main focus of her talk, or just one topic among many, but it has stuck with me utterly. She was talking about using two narrators in her book Catching Heaven and that one of the ways she made sure their voices were distinct was that each was using metaphors appropriate to her experience. The sisters had led different lives, so of course when they reached in their minds to compare one thing to another, they were drawing on different pools of knowledge.
My little mind: blown.
And I’ve used this insight in every single piece of fiction I’ve written since (which I can’t say about much writing advice!) Your metaphors have to match your characters. Whether you’re writing in first person or third, historical fiction or sci-fi, war or romance, a character can’t compare something they’re encountering to something they’ve never encountered before. The entire concept of metaphor is based on comparing to something within your experience. It can be easy to lose sight of that when you think of a clever turn of phrase or discover a colloquialism you find entertaining. Personally, I have been dying to use “ugly enough to raise a blister on a washpan” for years, but no character I’ve written so far would say that, and I know not to force it.
Similarly, if your character is using a metaphor from within their realm of experience — which doesn’t necessarily match your experience — it needs to strike the right level of detail. If your character’s a die-hard Giants fan, is he going to say “I felt like a quarterback making a touchdown,” or is he going to say “I felt like Eli Manning at the Super Bowl making that pass to Ahmad Bradshaw”? (But don’t go too far — he probably wouldn’t say “Super Bowl XLVI”, would he? Make sure your characters are still speaking like people, not mouthpieces for research. We call this the Crichton Conundrum.)
So keep a close eye on your metaphors. If you don’t, someone else will, and it might not be until after publication. And that’d make you feel like the kid in the corner in the dunce cap, wouldn’t it?