Spotlighting

Carl Spitzweg, "The Bookworm," 1850

In theatrical illumination, the light is almost never completely uniform. Less important areas of the stage fall into shadow, while the spotlight rivets the attention of the audience on the most important part of the action.

Achenbach

Landscape painters use the natural spotlighting of a cloudy day to dramatize selected areas. This device was common in the Düsseldorf School, and among 17th century Dutch painters like Ruisdael.

Capturing these effects while painting en plein air requires tremendous concentration and commitment. Once you see an effect that you want to capture, you have to paint it from memory and ignore other effects that strike your eye later.

Using a special eye light to spotlight the center of the face was common in film noir lighting.

Spotlighting can have powerful storytelling implications. In Gerome's "Death of Caesar," the body of the slain Caesar has already fallen into shadow, as if the light of history has moved on to his murderers and successors. 

Gradating the edge of the spotlight effect makes it less obvious to the viewer, but it takes a careful organization of the palette if you want to paint it with opaques, because all of the colors of the scene must go through gradual transitions.

Alphonse Mucha, "Nero Watching the Fire of Rome," 1887

If you can group the values of all the figures in the light and make them a mass of light, in contrast to a mass of figures in shadow, the overall composition will be much simpler and more powerful. 

This also takes a lot of concentration: always to keep in mind the large statement, while you're fiddling with the details.

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More about spotlighting in my book, Color and Light, available from Amazon (where it is still the #1 book on painting), and signed from my website (U.S. orders only).