John Gregory Dunne, from The Studio: 

The Studio was simplicity itself to write. It was mainly a matter of transcribing and rearranging my notes. That there were no surprises—I knew exactly what I was going to do—was for me the problem. Writing is essentially donkey work, manual labor of the mind. What makes it bearable are those moments (which sometimes can last for weeks, months) when the book takes over, takes on a life of its own, goes off in unexpected directions. There were no detours like that in The Studio. My notes were like plans for a bridge. Writing the book was like building that bridge. 

This chimes with my experience. The two books I most enjoyed writing are Original Sin and The Year of Our Lord 1943, because they posed serious structural challenges. It was not obvious to me how those stories might best be told.