Where, when, and who are the three primary components of scene-setting in a script. For example: ON ILLYRIA (who)... Words like these in a script suggest to a director what is important about the scene, and -- if the director agrees -- help to cause the scene to be rendered in the way the author imagined it would have been. Note that Where When Who might sometimes be where, when, and what, rather than who. An example from the same Angel script: CLOSE ON a white wall for an instant... Beseech the deity of your choice for mercy, O Writer, if the places are more important than the people.