Film is the synthesis of the arts. It's also its own unique art form. To be a good director you should know all the arts, because they all figure into what you do (or should be doing) as a director.
STORY -- you must know the classics in literature, poetry and theater.
IMAGE -- drawing, painting, printmaking & photography.
DESIGN -- architecture, fashion design, graphic and interior design
PACING/EDITING -- you can learn a lot from dance, opera, jazz and classical music (think of film as dancing images with words)
MUSIC -- you can learn from all musical forms, and you should know the basics of all types: jazz ,classical, rock, folk, world, country, rap, etc. And get a basic musical education -- know why chamber music is different from a concerto and what an oboe sounds like.
A director's job has three parts:
A) Having the vision (and being committed to it)
B) Communicating the vision (to your team).
C) Realizing the vision (on the screen).
Parts A & C are based on talent, heart and guts. And the more you know about the Arts, the easier Part B is to learn. You need to know Ibsen from Miller from Mammet to talk to your actors. You need to know allegro from diminuendo from legato to speak to your composer and music editor. You'll need to know Monet from Hopper from Weston from Rubens to talk to your director of photography. You'll need to know Dior from Queen Anne from Bauhaus to talk to your Production and Costume designers. You must make each department's aesthetic blend seamlessly with the others to make the whole damned thing one piece -- YOUR piece. Plus, you will win your crew's respect -- a major battle for a first-timer.
You'll also have to know the art movements and what they meant to their times. That means understanding the difference between Gothic and Realism and Mannerism and Expressionism.
The simple fact is that the arts can only be spoken about in the context of other artworks. You might have to say that the "mood is Baroque" or the score needs to be "more like Beethoven here" or that this line should be read like it "was written by Hemmingway". Directing is leading a ragged pack of artists on a perilous journey, and the first step is to help them understand what the hell you are talking about.
Besides all of that -- you will be infinitely inspired by what you learn from these painters and designers and musicians and writers. They all dealt in the same commodity, HUMAN EMOTION -- the very stuff you are dealing with when you make your movie. Why not pick up a few tips from the masters?
Read the next one!