Low-key lighting is a method that creates shadows and intense contrast of light and darkness. A Fresnel light is a focus-able light used in film that has a special lens which can be adjusted to focus its beam. By lowering the Fresnel light to its lowest collapsible position and aiming the light upward at your subject, you create frightening and dramatic shadows. The effect is the same as if the subject were to hold a flashlight beneath his face while telling a ghost story.
Attach barn doors to shape the light and create more contrast. Barn doors are fixtures you can add to your Fresnel lantern to direct and focus light. They create even longer and darker shadows, allowing the exposed parts of the subject to be very bright, and the covered parts of the subject to be intensely dark.
Add an eerie color to the light using color gels. Color gels are heat-resistant colored sheets that clip on to the Fresnel light. Use a red color to enhance blood, or a green color to add a sickly glow to the subject's skin.
Create a flickering effect with a practical, such as a candle, a lantern or another natural light source. A well-placed candle can balance the Fresnel light and add a spooky element to the scene.
While direct light and deep shadows are the most common lighting technique for creating an element of fear in films, diffuse light, aided by fog or mist, can likewise create such an effect. Bright, diffuse light won't work, of course -- it must be dim, and the fog or mist must be backlit to show. To create mist or fog, use a fog machine or some dry ice and fan it gently in the right direction.
An alternative to using Fresnel lights is the use of fluorescent lighting tubes to create a harsh, green-tinged light. This technique is useful for scenes that take place in public places that commonly utilize fluorescent lighting, like a school or a department store. Because the fluorescent lights flicker or pulse naturally, they can add an extra-spooky feel to the scene.