Films are instruments of storytelling.  Their methods are catered to the two of our most reliant senses and therefore have the greatest control out of any storytelling medium in conveying objective, rather than subjective, information.  In short, they leave little room for the imagination.  It is therefore absolutely essential for the storyteller (director) to be aware of and have full control over the tiniest detail in both the visual and the audial production.  Every word, colour, sound, shape, and note will yield a specific emotional response, which will in turn have a profound effect on the way the audience interprets the scene.