Adding life to a scene
While the purpose of the backing is, by definition, to remain in the background of the scene, there is always the danger that the static nature of the image can make it look unrealistic. To combat this, a variety of tricks and techniques can be employed by the Cinematographer or Art Director.
Small lamps, strategically placed, can greatly enhance a night scene. Tiny blinking LEDs can be attached to or through a backing to create the look of anti-collision lights on tall buildings or radio antennas. Gently pulsing lights behind a drop can also create the illusion of atmospheric distortion on a panorama of city lights.
Sunrises and sunsets can be projected upon a backing, as can a moon in various phases. Moving lights can also be projected on the front or rear of a backing to create the impression of waves on water, leaves gently blowing in the breeze, or even rain or snow in the sky. Even after the scene is in the can, digital effects can be used to add life to the backing. Digital smoke plumes or birds flying past a window go a long way towards selling the illusion.