fly-on-the-wall documentary in British English
(ˈflaɪɒnðəˌwɔːl ˌdɒkjʊˈmɛntərɪ)
noun television a documentary 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 fly-on-the-wall documentary about the Queen's life