Short version? Do it.
Long version? Use all the big words your heart desires.
Now here’s the tricky bit: making sure you don’t loose your audience. You want your readers to understand what you say. That’s the most vital part of writing. So, when you use the big, or unusual, words, you want to be understood. Like so many bits of writing advice, this one is so much easier to say than it is to implement. But I’ll do what I can to make it easier.
All you need to do, is the use of context clues. You want your readers to be able to divine the meaning of your fancy words without them needing to stop and pick up a dictionary. Not only are they unlikely to do so, it breaks the flow of your story as they blink down at your word and go, “wait, what?”
Context clues usually comes down to simply rephrasing the word in the same or a following sentence. Here’s an example of what I mean:
“The villain was downright diabolical. He had no respect for children or old ladies, and was frequently seen kicking kittens.”
Now, even if your audience doesn’t stop to look up diabolical to find out that it’s, “of, concerning, or pertaining to the devil” they still got the idea that it’s a bad thing to be.
This may seem simple, or even childish, but it’s one of those little things that can get away from us as writers as we get going. This way, you can pick the word that best fits your situation without worrying that you’ve lost them. You don’t speak down to them, but you also allow people who may not be familiar with the words to continue to read and enjoy your story.
A final bit of advice? Look up the word yourself before you use it. Because, if you’re anything like me, you learned the meaning of words through context clues. And as you see above, they’re great for getting the general meaning across, but things can get lost.