A ~ documentary is made by filming people as they do the things they normally do, rather than by interviewing them or asking them to talk directly to the camera.
...a ~ documentary about the Queen’s life.
a fly on the wall: see fly
ADJ: ADJ n
FLY-ON-THE-WALL
Meaning of FLY-ON-THE-WALL in English
Collins COBUILD. Толковый словарь английского языка для изучающих язык Коллинз COBUILD (международная база данных языков Бирмингемского университета) . 2012