Usually, those persons closest to the incoming President will be the main leaders of the Transition effort. They are most familiar with his policies and practices, and are able to interpret his wishes regarding the structure and staffing of the new Administration.
I suppose we all loved those kind of sci-fi movies where terrible things came out of swamps and came to Mars. And there's usually some poor girl. All the guys are trying to desperately handle levers and saying, go to something or other.

