Language: you need to know how your characters talk
How will you know how your characters talk if you don’t research it? This most obviously applies to historical fiction, where anachronistic language can kill a novel’s claim to historical authority, but it applies to every other form of fiction too. Got a character who’s a scientist? Better learn your periodic table and some basic science, because chances are that character is going to be slipping into science-speak at some point. Is your novel set in the Bronx? You’d best look into regional dialects and vocabularies. It’s all about ensuring authenticity.
You get the idea—you have to know how your characters talk; what language they use; and any slang, colloquialisms, and dialects unique to your setting.