There are some out there who believe that writing is just writing. If you have training in writing of one nature, surely you can write other stuff too. Well...
There are certain aspects of writing that hold true no matter what type of writing you do. The rules of grammar, for example, don't care if you write fiction, a scientific paper, or a cookbook. However, there is a massive difference between all three of those particular types of writing.
It is becoming increasingly common for those who have nonfiction writing backgrounds to shift into the fictional realms. Let's face it, we have big imaginations and we want to share that with the world. Rightly so. Our stories should be told. However, nonfiction writers, you need to retrain your brains if you are serious about pursuing fiction.