A little knowledge is said to be a dangerous thing, but what about too much knowledge? If you’re unsure, just pull up a chair next to a Star Wars geek and ask, “So, Tie-fighter or X-wing fighter—which is better in a dogfight?”
This is the tricky thing about realism. Readers today are sophisticated. They won’t take things at face value anymore. You need to be pretty sure of your facts before you commit them to paper, so being an expert can have a lot of advantages. If you’re an oncologist, who’s going to fault you on cancer treatments? If you’re a cop, who’s going to pick holes in your investigation techniques?
That’s the upside. What about the down?
As an expert, it’s easy to wax lyrical about your specialist subject and fill page after page with detail that only an expert would know. This makes you sound like an authority on the fictional and factual matters at hand, but God, are you dull. Suddenly, you’ve turned an exciting novel into a textbook (are you taking note, Tom Clancy?).
Sometimes it’s good to be an outsider to a subject. You can rationally decide what is relevant and what isn’t. When you’re an expert, it’s not always easy to remain objective and you have to walk a fine line. It doesn’t matter whether you’re a nuclear physicist, a cop, a pathologist or even a seamstress. Give us insight, not a lecture.