"Write what you know" is the advice most often given to beginning writers. If you know about something, how hard is it to put what you know down on paper? Maybe you know more than you think you do ... and again, maybe not.
Nonfiction writers expect to research their subject. Why should fiction writers be any different?
"The devil's in the details. . ." That says to me that if you want something to be credible, whether fiction or nonfiction, you need to be sure of your facts. That means research them, double-check them, and don't expect to slip something by your readers who just may know more than you do about a subject!
Finding Papa's Shining Star just went back to my editor this morning. The conflict between the two lovers is that one knows who he is, and the other just thinks she does. David, raised in a far-from-orthodox Jewish home, returns from World War II after spending a year and a half in Dachau because of his Jewish heritage and the fact that he speaks fluent German. The months spent trying to survive the concentration camp have heightened his awareness that he is a Jew and now feels a responsibility to be part of the new homeland of Israel.
I did a great deal of research to make sure that the historical timeline and references to Shabbat, the Hebrew language, synagogues, and so on were accurate. Still, I knew that my protestant interpretation of what I read could lead to problems. Fortunately, I found a fellow author who could answer my questions about some of the details I wasn't convinced were 100% accurate. I researched--and then I double-checked.
To me, it's about respect for my subject and my readers. Credibility is everything. I've "tuned out" on books where I've found misstatements.
Bottom line: when writing what you know, be sure you know what you write!