What action is performed to increase or decrease the intensity of lighting?

The action that is performed to increase or decrease the intensity of lighting is known as "Fade up/down." This technique involves gradually adjusting the brightness of the lights, either by smoothly increasing (fading up) or decreasing (fading down) the light levels. This creates a more dynamic and controlled lighting effect, allowing for shifts in mood and focus during a performance.

While other options are related to lighting in different ways, they do not specifically describe the process of adjusting intensity. Crossfade, for instance, refers to transitioning from one light cue to another without a blackout. Focussing is the act of adjusting the direction and sharpness of the light beams, which does not inherently change their intensity. Gel refers to color filters used to change the color of the light, which also does not affect the intensity of the light itself. Thus, "Fade up/down" is the most accurate answer in the context of adjusting light intensity.