When it does not, the camera gets fooled. Brightly lit faces against dark backgrounds can also cause problems. Another solution is to use a different angle where the light is more uniform, or where the subject is better lit. Windows and open doors to one side of the subject often provide quite nice and dramatic lighting. Be careful not to meter the light from the window or door itself, though.
Exposure is a combination of film speed, shutter speed, and lens aperture.
For any given amount of light and film speed, there are many possible combinations of shutter and lens settings, all of which allow the same amount of light to reach the film. Your decisions about shutter speed and lens setting will affect how much and which parts of your picture are sharp. Shutter speed determines the amount of blur that will be caused by motion. There is a trade-off between the two. The faster your shutter speed, to better stop motion, the wider you must open your lens, thus reducing depth of field.