The reason for doubles is so that the actual person doesn't have to be on set all day and waste their time, so they get doubles who are about the same height/build/hair style etc and for the shots that you cannot see their face it is usually the double and for the shots that are in front of the camera it is the actual person. The double also usually stands in until the director gets the shot perfectly lined up and the way they want it, then the actual person steps in for minimal work, again not to waste their time