Controlling light to create a specific look or mood is one of the most enjoyable parts of my job. Shooting on set is ideal, but shooting on location at any given time of day means you need to know how to manipulate, downplay, enhance or block whatever is naturally there. If you block it or downplay it, you need to know how to replace it with what you want.
That means having a clear vision in your head is essential. Visualizing the look that you or a creative director wants is the first and most important step to creating a particular mood.
Not good at visualizing? Work on it. One of the most important things I learned in Fine Arts school was: trust your eyes. Do blind contour drawings. Paint. Do whatever it takes to become excellent at observation. Sometimes we presume things, but when you really force yourself to look, you see the reality of what light actually does.
For instance, direct light from the sun creates parallel shadow lines. But physically, light is made of waves and particles. Therefore it can also bend and wrap around slightly - especially when diffused. See how it ricochets. See how its hue changes as well as its angle as the sun shifts through the sky.
After becoming intimately aware of those things, understand how your modifiers can replicate the same effects.
In developing a mood for your client, you might want to push for a "pre-light" session.
Light has a lot to do with branding so it's important to understand what your client needs, visualize then nail it.
