Having a basic idea of what your movie needs musically is a very important thing when you enter the dialogue with the composer.
There is that common misconception between directors that composers are happy when you just let them do their thing without giving much directions. However rather the opposite is the case. In such situations your composer gets thrown into cold water. Many movies offer the possibility to approach them musically in very different ways and for the composer it feels very uncomfortable to be shooting in the dark to see whether one might find something that you like.
Basically, you should think about the following things before you meet up with a composer.
1. What style of music do you want? This means not only in the big scale of “orchestra vs. rock band” but also in the smaller scale like “A big lush hollywood orchestral sound vs. an intimate and small acoustic score”. There are a few standards regarding certain movie genres and styles and while trying out new things is a great idea, trying to re-invent the wheel might not be. Trying to score a drama with minimalistic electronic music might work on a few occasions but more often it doesn’t. There’s no problem when you are not entirely sure about which style but at least go into discussion with the composer with a few possibilites that you could imagine working with the movie and discuss with him/her about these.
2. The amount of music. A few genres are quite self-explanatory regarding this (e.g. fantasy movies with very much music) but quite a few movies can be approached quite differently there. Deciding for less music usually gives more of a “real” impression while more music creates more of a “cinematic” feeling.
3. Specific music moments. Things like main titles and montages are usually the perfect spot for music to come in and take over the dramatic lead. If you have such moments in the movie you should think about those and what kind of direction you want the music to take them.
4. Important scenes and twists. You know the storyline and the way it goes best in your movie and you might want to see certain connections between scenes or highlight them in order to further sharpen the dramaturgy of the movie. These are important scenes for the music and you should have a vision about if and in what way the music should work there in favor of the overall dramaturgy.
These are just the most important things that you should be prepared for before meeting with the composers. It of course helps alot to also have ideas about “less important” scenes but you don’t want to be sitting in the dialogue with the composer and being asked something like “Ok, in that scene here, I have the feeling it connects somewhat to that sub-plot a bit later, do we want a reference there?” and you actually need to start thinking about that at this moment.