American illustrator Austin Briggs (1908-1973) says: "When working out an idea for an illustration, it is essential to keep in mind the shape of the finished painting at all times. Otherwise you are likely to develop a composition that simply won't fit the picture space."
"I sketched the idea with great satisfaction, thinking I had the answer to the picture problem."
"When it came to working out the rough, however, I soon realized that this idea could not be made to conform to space requirements. It was very difficult for me to get this approach out of my head and find a design that would work. If I had kept the shape of the picture in mind from the start, I would have discarded this idea before I became too attached to it."
From Austin Briggs' Master Course from the Famous Artists Course, 1952
Austin Briggs on Wikipedia
Thanks, Matt Dicke