The beginning filmmaker may find the prospect of working with a composer to be a bit intimidating, and that is understandable. However, there is no shortage of composers out in the world that are eager to work with filmmakers to create a score for a short film. Hundreds of them abound on any given film related message board. Most will work for free or very little, and some are very skilled, talented and experienced.
It would behoove the director to know the cues in the scene and have an idea of what they want to accomplish with music in the scene. However, for the director who is not music savvy, much of that visualization can be handled by a competent composer. A composer worth their salt should be able to identify the cues in the picture and read the emotional content on the screen and create music around that emotion.
The most important thing to keep in mind is that each composer has their own style. It is unreasonable and unwise to ask a composer to create something that is outside their repertoire. For some pieces, especially longer form works, a moviemaker may need more than one composer if they wish to utilize different styles of music. That is really the critical decision that the director must make: they must know the style of music they wish to use and choose a composer that matches that style. The easiest way to do this is to listen to samples of their work to see if it will work for your piece.
If finding a composer is beyond the scope of the project, a viable alternative is creating a score with loop based samples. A wide range of software is now available that allow the moviemaker to create multi-track music in sync to picture. Some, like the popular Sound Track Pro from Apple, come with thousands of samples that can be used royalty-free. These samples are loops of instruments (i.e. guitar, drums, piano, strings) as well as synthesized sounds and beats that can be used to construct themes. Creating a basic score for a short film can be done in a few hours, and it will add enormous value to the project. One drawback to this approach is that the samples from popular programs end up being used over and over again, and one need only attend one or two short film festivals to hear these same sounds in more than one movie. However, that is only a pitfall of the lazy as the internet abounds with royalty-free samples that can be imported into most of these programs.
With these kind of tools, there is no excuse not to have cue based musical elements in your sound track. You may start with your own loop based musical elements but a composer will add enormous value to your project for next to no cost and little effort. It makes very little sense for the filmmaker not to work with a composer, and the sooner one makes that step the better.