The human eye is very sensitive to lines, real or imaginary and the use of lines is a good way to lead the viewers eyes towards what you want them to see.
There are three types of edges or lines, an actual line, an implied line, and a psychic line. An actual line is the easiest to recognize because it usually defines a shape. A line is implied by positioning a series of points so that the eye tends automatically to connect them. A psychic line is not a line which is seen, but one which is felt as a mental connection that exists between two elements. The eyeline of a character is a good example. If a character looks toward an object, the viewer will also look. These psychic lines are very powerful and can be difficult to minimize if they are distracting.
A line that leads to an object, but doesn't quite reaches it, causes a visual tension that will draw more attention than if it were to touch the object.
Horizontal lines imply stability, vertical lines imply potential motion, and diagonal lines imply dynamic motion and depth. In the rectangular cinema format, horizontal and vertical lines work as stabilizers and reduce feelings of movement since they mirror the format boundaries.
The careful placement of lights to create shadow edges is one way to create lines and contours.