Say Amen, Somebody, a 1982 documentary about gospel music directed by George Nierenberg, is the only movie that I would qualify as viewing essential to properly appreciate a particular style of music. There are a few others that come close: The Last Waltz comes to mind, but its subject is a band (the Band, actually) and not an entire genre. This film not only acquaints viewers with founding figures such as Thomas A. Dorsey, and great performers including the Barrett Sisters and Edgar and Edward O'Neal, but it also presents with amazing directness the feelings of involvement and community that are central to gospel music. And even though gospel has been borrowed from by virtually every major r&b artist (starting with Ray Charles), and by artists of other styles as well (jazz, rock, and country to name three), it's rarely received the musical respect it deserves. Not by people who've seen this movie, though: http://www.youtube.com/watch?v=nLgk0WMI1-Y.