There's a fine line between writing fiction and nonfiction, but one of the things that distinguishes a great writer of both is the ability to know what to leave unsaid. After all, truth is sometimes harder to believe than fiction.
Best-selling author J.A. Jance learned that when she was shopping around her first attempt at writing a book. As Truman Capote did with In Cold Blood, she attempted to add elements of fiction to the true account of some murders. The editors who turned her book down said the parts that were real were unbelievable, but they were enamored with the fictionalized portions. She's been writing fiction ever since.