I begin with my usual comment that you can find more than you need about photographic exposure (luminance, incident light readings, exposure values, 18% reflectance, grey cards, the zone system…, HDR, etc. etc.) on the web. My aim is to focus on the stage situation, but I shall give a brief & annoyingly vague definition of my own.
A correct exposure is one in which the optimal amount of light from the most important elements of your composed shot reaches the camera sensor or film, whilst also respecting the dynamic range of the subject from brightest highlight to darkest shadow.
A correct exposure is a balancing act between the triumvirate of shutter speed, aperture and ISO. There is a reciprocity between them. Varying one requires compensatory settings in one or both of the others.
Which has the highest priority? It depends upon the current shot: I might need a smallish aperture for more depth of field to keep actors both upstage and downstage relatively sharp; I might need a fast shutter speed to ensure actors’ motion is not too blurred; or I might need a lower ISO value for higher quality (low noise) images for subsequent large size reproduction. Typically the shutter speed/aperture balance changes from scene to scene.
I don’t seem to get many brightly lit productions. Maybe running a website called Darkling Images has something to do with it. As a consequence I often don’t get much latitude in aperture and shutter combinations. Sometimes aesthetics come second to just capturing an image. Settings such as ISO 6400, an aperture of f2.8 and a shutter speed of 1/15 second are not uncommon. Depth of field can be quite small and the risk of motion blur quite large at these values. At least noise—with a full frame sensor and ‘proper’ i.e. adequate exposure—is not too intrusive. However, too little light and the consequent under exposure mean that the noise will leap out of the shadows and smoother you in a speckled slush?