When we think of research, in context of writing a book, we immediately think of medical papers and historical documents that can be used in the creation of a nonfiction book. However, it is important to know that research is indeed applicable to the other realm of work when writing: fiction. In fact, I can tell you multiple reasons why this is so important. Let's take an example with a book I just finished reading, All the Ever-Afters. (You can read the book review for it here!)
This is clearly a fiction book: it deals with Cinderella's stepmother as the protagonist and documents life through her perspective. Now one might think, what could we possibly need research for, in stories like these. Think about it this way: wouldn't you want to read something that seems realistic in all aspects? In setting, clothing, or character? These are, might I emphasize, the true elements that carry a novel, and with it, credibility. As an author, credibility and trust is something you must establish early on in your book, fiction or nonfiction. Essentially, it's what keeps your readers hooked.