In the “About the Book” postscript in her 2007 novel Run, Ann Patchett includes snippits from an interview with Jonathan Burnham (publisher of HarperCollins)
Jonathan Burnham: Was the act of writing nonfiction at the same time interference, or did it help clarify things for you? Or did it make no difference whatsoever?
Ann Patchett: It really made no difference. Writing nonfiction is so much easier than writing fiction, for me, that it really is akin to saying, “I did my shopping list and then I went back to my novel.”