Motifs can act as comparison devices, if they look like something else in the film. Thus, they take on an added symbolism. This can act as a very emotive device - if Superman spots something that, at a glance, looks like Kryptonite, then we also wonder what will happen until we are reassured that the object is not. Thus, the parallels that motifs can create can be very striking in viewer emotion. Difference Reiterating the same devices over and over in film becomes monotonous and ultimately tedious, as does similar visual and verbal product.
Differences produce interest and contrast.
Again, difference works very emotively when it is manipulated. If the film is seamlessly woven together, it has unity. Disunity can be as small or as large as the director wishes. Progression of form Mainstream entertainment cinema allows for progression of form to keep the pace fast and tense. Consider a synopsis of a film - there are major parts in it which are foundations of the story.