By simply eliminating ALL outside light sources, you put the focus back on the stage where you want it to be. Even a basic 'single floodlight' set-up can be improved dramatically by simply darkening the surrounding area. Flashlights can be modified to become pin spots or even follow spots. A pin spot is a fixed light source that only illuminates a specific area of the stage. Usually this is used to accent one performer during a monologue or other break from the main action.
Singers often employ pin spots for solo singing.
In a professional set-up, the pin spot is usually suspended on a framework above the stage and is carefully focused on the desired area by means of flaps surrounding the bulb. Some pin spots are also controlled by pulling a focal lens. You can achieve the same effect by bracing a strong flashlight above the stage and controlling the available light with a cone of paper. Darken the stage area entirely, then move the flashlight until it illuminates the intended area. Brace it with duct tape or a c-clamp to a sturdy structure.