So the books will often lead off with a description of the various kinds of lighting instruments. And often a short dissertation on Ohm's Law et al.
Naw; I'm going to lead off this brief overview of the subject by talking about the kinds of light. More than anything else, theatrical light is about control. As important as it is to put lights on actors, it is equally important not to put light on random walls, audience, bits of backstage. Check out a theater set some time under work lights; the effect is quite different! Theatrical lighting starts by bringing in light just to those elements we wish to reveal.
That's why almost all of our tools are directional. But even then, they fall into two rough groups; those that are just directional, and those that can be even more tightly controlled. The latter is the ellipsoidal, often called generically an "Ellipsoidal Reflector Spotlight" or "ERS", but also typically named by brand and/or size.
It is that reflector that gives the thing its peculiar quality. The reflector focuses light, you see. Brings it into a tight focus point within the housing of the machine. This allows the light to be precisely masked to keep it away from distracting spills of stray light, and even allows the light to be textured or patterned by placing steel-cut patterns into that same focal plane.
This also makes these instruments more expensive and a slight bit less efficient. This is why they are found side-by-side with the other work horse of the theater world, the fresnel. Technically, an instrument with a hemispherical reflector and a fresnel lens, but what this means is the light never falls through a focal point. It remains soft and fuzzy through the throw of the instrument.
Of course, both tools can take on some qualities of the other. The fresnel can be modified with Barn Doors or other devices that more softly mask off unwanted spill. The ellipsoidal can be thrown out of focus or (preferably) softened by placing sheets of diffusing material in front of the lens.
But these two general types really does cover the kind of tools you have to create the lighting look desired. Par cans, cyc strips, scoops, follow-spots, intelligent lights; all of these can be considered variations of one of these two themes. Powerful, diffuse light, or tightly controlled, focused light.
The qualities of light, they are many. Color, intensity, angle, texture. And even within these, the light from a Par Can is subtly different from the light from a fresnel, giving a different aesthetic quality (as well as different technical considerations).
But before we get to that, we have to recognize that theater is an artificial environment. In the real world, there are often hundreds of different light sources -- or sources with innate qualities that can't be properly simulated, such as the horizon-to-horizon glow of the daytime sky, or the sheer power and starkly parallel rays of direct sunlight.
Too, theater is, well, theatrical. Like every artificial recreation of experience, it is selective and amplified. Where in the real world the glow of the candles on a birthday cake are but one of many elements, to reproduce the aesthetic of our perceptual experience we need to reduce the picture to just that glow, and perhaps a little extra to define contours, provide a background, etc.
Human perception is filtered by our attention. The power or contrast of a birthday cake only plays correctly at living room distance. Across the void of fifty feet of audience seating, we have to exaggerate in order to properly mimic the effect.
Plus, of course, we don't have access to the full panoply of the world. In this cockpit we can not hope to hold the vasty fields of France; we can only suggest them with some earth and a tree or two. Especially when the lighting is asked to create dozens of different environments, you don't have the luxury of full simulations. You must make do with sketches.
Lighting, of course, is able to make use of an established language. You can light the actors from behind with bright blue and the audience will accept the conceit that they are supposed to be in pitch black. You can cast the shapes of light through leaves and the audience will accept that there is shrubbery around (even if they can't see it). The bright circle of a follow-spot is perfectly accepted as belonging with a sprightly song, even if it no more matches any non-theatrical human experience than the way random passer-by's suddenly break into flawless choreography.
So, aesthetically, one of your goals is to recreate the feel of a lighting environment (or produce one that is interesting to view.) Some are as obvious as the flickering neon sign outside the seedy detective's office (or the way light falls through the slats, casting a zebra-pattern of shadows on the detective's face). Many are not.
It is tempting to think in terms of diagetics here. That a night scene under a streetlamp can be lit as if that streetlamp is a primary source. Or an in-one (aka, a short scene played in front of the show curtain, with minimal or no scenery) can pretend to that same kind of environment, up to and including the yellow pool of a streetlight that is not being provided by scenery.
And other scenes, light simply exists because it needs to be there; it isn't directly representational of anything within the world of the play.
In my experience, the trick with these sorts of effects -- where the look is dominated by one strong element -- is to provide balancing elements to fill in what would otherwise be dark spots. These may or may not be equally representational.
To return to our streetlight example, pair the pool of yellow light with a cool backlight that can variously be taken as reflected light of the city, or moonlight, or just a general "the light of all dark places." But since both of these angles are high on the actor (and one is behind her) we add one more angle; something unmotivated but necessary to light up that all-important face and expressions just a little.
This works best, as I said, when you bring in just enough to make the total effect pleasing, but never so much you wipe out the strong presence of that element you've chosen to emphasize.
Technically, figuring this out looks a lot like a color wheel. You will usually want to take a ruler and bring your supporting lights in at a significant angle to that first light. And you will want to chose their color (and, often, their quality) to complement it as well. 45' is a good rule of thumb in the horizontal, angle to the audience permitting.
This significant angle works in the colors, too. Against that cool backlight, the face light should be a warm, or at least a neutral lavender. The closer your colors cluster, the more you carve out a distinct corner for this micro-look.
Of course this same scheme easily expands to cover the stage, as I explained in an earlier essay. Once you've established a balance of the various lights entering a specific snippet, you can copy those lights over and over again across the width of a stage to create your basic plot.
But here's another division I didn't touch on in the previous essay. Some scenes will make use of the whole stage width, and thus the lighting for those is derived from your area plot (plus whatever use you can make of color washes, gobo washes, and the like). Other scenes are more restricted in stage space. They do not need the entire stage. And this is extremely useful, because these smaller scenes can become micro-plots. You don't need to re-create the "man under a streetlamp" across forty feet of stage if there is only one lamp and one man.
If many of your special needs (in terms of different locations, emotions, times of day, etc.) are also small, tight scenes, you can get by with just one overall look and a handful of specials to cover the oddballs.
And a few specials go a long ways. If you can bring out something the audience hasn't seen before at least twice during the length of the play, you will make the audience feel as if the lighting had variety to offer.
And, yes, there are also some scenes that are peculiar in their own ways -- an extended chase scene through a darkened garden, for instance, or perhaps a dream ballet, or a storm -- where you do have to cover the entire width of the stage but you are relieved from having to light it well. Look out for these scenes where you can pull up a wash or a slash of light or something else peculiar -- and not have to hang twenty instruments to cover the whole width of the stage in smooth pleasing front light.
Lastly, never forget that lights are dynamic. The human eye is constantly adjusting its expectations, its acclimation, its white point. If you really need a scene to read as bright, work on the scenes around it to make them subtly dimmer. Everything is perceived in terms of contrast; to what was seen before, to what is seen around it. Given an entire act to pull the lighting down into the rose-colored shade Blanche DuBois asks for, a single naked 100watt lightbulb is blinding.