some notions from that book:
A documentarist must know perfectly the subject matter - seeing, experience. (Robert Flaherty - Nanook of the North 1922). The experience is very important. The author's fascination of the subject matter is somehow projected into a form of a documentary. The documentary should evoke the same feeling in audience. It is an essential assumption for an imagination result which should call out catharsis in audience mind (edification, exoneration, relief).
Once an author deeply knows ambient (background) he can structure a theme because the subject matter - seeing reality is not a theme itself. In general, a theme is an author's idea which is formulated on a base of particular observation and experience of subject matter from an artistic point of view. (Robert Flaherty's theme in general: Humanistic theme of human dignity, sociability, solidarity and greatness. Luis Bunuel: His general theme was about human abasement, human dignity humiliation by physical or spiritual affliction etc.)
Both Flaherty and Bunuel fulfil the cathartic process through a strong aesthetic and emotional experience. Besides positive, enjoyable and noble things of the subject matter there should also be those dark, tragic and negative sides of life but it does not mean that the art is negative. (Aristotle and catharsis).
A thematic limitation of art creation within an area of positive life values with a recall on their noble function is a nonsense because the creation loses its philosophy depth and life trueness.
If the theme is an expression of an author's idea in general than the subject is a content concretization.