Three-point lighting is the most basic lighting technique for film and photography. It consists of a key light, fill light, and back light.
Key Light: This is the main light. It is usually the strongest and has the most influence on the look of the scene. It is placed to one side of the camera/subject so that one side is well lit and the other side has some shadow.
This is the secondary light and is placed on the opposite side of the key light. It is used to fill the shadows created by the key. The fill will usually be softer and less bright than the key.
Romcoms tend to have even lighting with the fill almost as bright as the key. Darker films tend to make the fill less bright, adding more shadows and depth.
The Proposal (2009)
The Girl With the Dragon Tattoo (2011)
The back light is placed behind the subject and lights it from the rear. It provides definition and subtle highlights around the subject’s outlines. This helps separate the subject from the background and provide a three-dimensional look.
Back light can also be used to the extreme. When the key and/or fill lights are removed, a back light makes a subject silhouetted.