For some, this advice might bring their writing to a screeching halt. They fear that their lack of knowledge of firearms, history, wine-making, cars, or whatever it is they want to include in their story will be glaringly evident and thus they lose the trust of their readers. And while this is a concern that is justified, there is one even worse....
The writer who is so enamored of their knowledge that they have to share it. ALL of it. Every detail. Whether it is relevant to the story or not.
I write murder mysteries and I claim no personal knowledge or experience in committing crimes (that's my story and I'm sticking to it!) nor investigating them. However, since my characters include police officers, I have to have some passing knowledge of police procedures and weapons. This includes firearms. Now I have to confess, I know nothing of guns and have never handled one... until yesterday.
And now I know... and thanks to my friend's seemingly inexhaustible knowledge of firearms and their history, I also know more about guns that I'll ever need in the course of my writing career.
Does that mean that knowledge is wasted? Not at all! Of course, I was raised by a dad who insisted that it never hurts to learn something you don't need to know. But now I can write with confidence that if a person who never handled a firearm suddenly grabs a .45 and fires it, they're not going to act like it's no more difficult than firing a water pistol... trust me, those .45s can KICK!
So while I will use the knowledge I gained to make my fiction more real, I will spare my readers the fascinating, albeit irrelevant, tidbits of history and trivia that I picked up in the course of an afternoon spent on a gun range... a place that, 25 years ago, I assure you I would have never dreamed of being, doing something I never dreamed of doing. But that's the beauty of "write what you know"... it opens up a lot of possibilities to keep learning!