Have you ever heard the phrase jack of all trades, master of none? Do you know what it means? Most do but if not it simply defines a person that knows bits and pieces about many subjects but is not an expert on any particular topic. It is a saying that applies to almost every writer, especially fiction writer, that I have ever met. We all know a little about a lot of things but few of us could take the stage and give a seminar on any of them.
This happens because we get inspired to write books that either center around a hobby or profession we have never done or have a character that happens to be involved in something that has never crossed our paths before. We don't want to abandon our inspiration so we have just one choice, we have to become well versed on whatever subject we are currently clueless about as quickly as possible. We have to do research.
It is very similar to writing a paper in school. We start with the basics of what we already know then make a list of the deeper parts that we have to look up. It could be clothing type or speech patterns for a different time period. Perhaps we need a map of a location that we have never visited yet our characters are determined to live there. What if you are writing a medical thriller but your only experience with doctors is from your annual check up? The terminology as well as knowing what medications or procedures would go together, have accurate side effects and make sure everything is performed properly by the correct people.
I myself am working on a mildly erotic thriller series set during the same time as the Salem Witch Trials. There are a number of overlapping elements from astrology to Wiccan traditions that must be referenced. I also have to blend enough of the time period language for it to feel accurate while simultaneously keeping it light enough for modern readers to follow the story without getting bogged down. Also I must keep everything organized making sure I acquire all of the same information for each book in the series so that when a reader moves from one book to the next the story will flow the way it should. Research can feel boring and overwhelming but it is a sign of a professional writer to make sure you have everything in order so your readers can happily fall into your created world without having to Google something every couple of pages or question the authenticity.