Positioning the Fill Light
The fill light source is often positioned directly across from the main light source to simulate light reflected from the environment. Since this light is more often than not a diffuse light, i.e., its location cannot be defined by the viewer, it should not cast a shadow or generate a Specular highlight. The Area light is also well-suited for use as a fill light.
As the image below demonstrates, the fill light can be placed facing the side opposite of the main light source. This will brighten the side of the car opposite the main light source and brighten the very dark shadow created by the main light. Depending on the complexity or shape of your object additional fill lights may be required in order to create realistic shadows.