In modern photography, the fill-in light is used to control the contrast in the scene and is nearly always placed above the lens axis and is a large light source (think of the sky behind your head when taking a photograph). As the amount of light is less than the key-light (mainlight), the fill acts by lifting the shadows only (particularly relevant in digital photography where the noise lives in the shadows). It is true to say that light bounces around a room and fills in the shadows but this does not mean that a fill-light should be placed opposite a key-light (mainlight) and it does not soften shadows, it lifts them. The relative intensity (ratio) of the Key-light to the fill-light is most easily discussed in terms of "Stops" difference (where a Stop is a doubling or halving of the intensity of light). A 2 Stop reduction in intensity for the Fill-Light would be a typical start point to maintain dimensionality (modelling) in a portrait (head and shoulder) shot.