I’m really not sure what it is, but it’s like when a person sits down to start writing a book, they think they have to elevate everything. Suddenly, exposition and dialog are both nothing like anyone ever really speaks. Even in a contemporary story, everything the characters say sound like they are trying to live in a Dickens novel.
Stop it, people.
When you write a book and every other word looks like you had the thesaurus open on your lap while you were doing it, it has the exact OPPOSITE effect of what you were trying to achieve and you do not sound more intelligent as a writer. You just read like you’re trying way too hard.
Even when writing epic fantasy, trying to sound “archaic” in everything you write usually backfires too and just makes your story too dense to understand. Readers don’t want to fight their way through your book. Especially in dialog, don’t overdo it. Read your dialog out loud and think, “Does this really sound like how someone would talk?”
Sometimes plainspoken prose really is not an evil, but a way to convey your point and actually make it get through to your reader.