In situations where there are practicals (light sources visible on-set) involved, such as a window or lamp, it is best to use that source as a "motivation." This means that you will light your scene to appear that the visible light source is supplying most of the light for the scene. For example, if you have a window on one side of the subject, you would avoid putting a key light on the opposite side of them. If the window does not provide enough light, or is uneven, then you would add a little more light from the same side as the window. On the opposite side of the subject, you would use only fill lighting, which could include bounce cards or very soft sources. Either way, in the end you want the shot to look as if the window is providing the primary light for the scene. In this way, the viewer gets a more believable image because the lighting fits naturally with its surroundings. Use common sense, of course. A dim lamp that is only used as a background element probably will not be the primary source in the scene. However, it might be good if your backlight hit your subject from the same direction as the lamp.
