The subject matter of any film you choose to make is doubly important because the process of filmmaking is such a relentless, tedious, and difficult experience that you have to be sure the theme is something that will sustain your interest. No matter how tough things get, or how discouraged you may feel, you have to be sure that morning after morning you’ll wake up to a subject that still fascinates you. It should preferably be a subject you don’t quite understand, even better if it’s based on a question you don’t know the answer to. Then the making of the film, which ultimately winds through several years, can provide an answer, which will be the film itself.

—Francis Ford Coppola, in his introduction to Mircea Eliade's Youth Without Youth of which he made a movie