When your film professor asks you to write about film, it's precisely those "invisible" aspects that you're expected to see. As "Looking at Movies" advises, you need to pay attention to the way the camera moves. Observe the composition (the light, shadow, and arrangement) within the frame. Think about how the film was edited. Note the sound design. In short, consider the elements that make up the film. How do they function, separately and together? Also think about the film in the context of when it was made, how, and by whom. In breaking down the film into its constituent parts, you'll be able to analyze what you see. In short, you'll be able to write a paper that transforms your thoughts and responses into writing that is appropriately academic.
This classic study was first published in France as La Semiologie en Question. A filmmaker and theoretician, Mitry asks if cinema is a language, can it be understood through the techniques of linguistic analysis? In effect, he interrogates semiology by representing its basic propositions and approaches, comparing them with scientific humanist aesthetics. Mitry's study ranges across film language, its syntax, grammar, and code; signs and signification; montage; images; narrative structures; symbols and metaphors; and the rhythm of film.