First you need an encyclopedic knowledge of all types of music, here's a partial list: Gregorian Chant, Baroque, Classical, Romantic, 20th Century, Folk (all types), Blues (all types), Jazz (all types), Rock (all types), Pop (all types), R&B (all types), Country (all types), Third World (all types), Ethnic (all types).
You'll need to know where to find the owners of the rights to all of this material, how to negotiate their use in the film, the films advertising and the soundtrack album.
You'll need your encyclopedic knowledge to find a different song that will satisfy the director when you can not agree on terms for the directors first choice.
You'll have great relationships with numerous composers, be able to communicate what the director wants for the score and be able to tell them that everthing they did was wrong and that he/she has to start all over again without bruising their feelings and still keeping them passionate about the project.
You'll have to know arrangers and conductors and musician contractors if the director wants a new recording of a piece of music.
You will have to have the patience to listen through hundreds of hours of unsigned material from indie music sources, develop the talent to listen to 30 seconds of a song and know whether or not it's worth listening to the rest and keep the "winners" catalogued in a library and in your mental file cabinet for instant recall.
These are the basics. Become or work with a DJ to learn how, when, where and why audiences react to certain songs. Work for a licensing house. Take some law courses. Take some psychology of music courses. Take some music history courses.
This was all advice given to a buddy of mine who wanted to get into the field. They gave him a "needle drop" test, which he failed miserably. He knew the songs, but could not identify the actual genre, such as the difference between smooth jazz and blue light jazz, or Hip-Hop and Rap, or Chicago Blues and Delta Blues; or would be unable to identify the composer or year.
Good Luck!
