The documentary film Night Mail, propagating the services of the British General Post Office in the 1930s, reflects the political ideas, typical of the period, of increased democratisation and a collective work ethic. At the same time, it reflects those advanced aesthetic ideas of the period that favoured a public function of art and saw art works as products of collaborative creative activity by artists from different media, with film taking a leading role in the coaction. The g.p.o. Film Unit was able to engage W.H. Auden and Benjamin Britten as contributors to the formation of Night Mail. This paper argues that their cooperative efforts turned the film into a unique case of what it calls a “collaborative Gesamtkunstwerk” showing a particularly close interaction of picture, words and music, and analyses the effects which the closeness of interaction has on the working of the participating media.