Donald L. Vasicek
Juxtaposing in writing and screenwriting is most effective if you think in terms of what film is. Film is a visual medium. Therefore, the writer/screenwriter must write visually. Writing visually includes striving to “show” rather than “tell” the story and the characters. The most powerful way to do this is to utilize the mingling of opposites or a dichotomy.
For example, if you have a character who represents evil in your story, then juxtaposing your “hero” or main character in your story to this kind of “villain” [in fiction, villains do not have any redeeming characteristics while the antagonist in your story should have, at least, 1 redeeming characteristic) should represent good, the opposite of evil, and thus, a dichotomy.
In summary, the most effective use of juxtaposing in movies, is what is totally opposite of each other, whether that is characters, objects, locations, etc. This kind of juxtaposing creates conflict, and in fiction, if there is no conflict, there is no drama, and the resulting effect of that is a flat story with no energy. Some would call this kind of story/film/movie, boring.
I hope this is of help to you.
Olympus Films+, LLC/The Zen of Writing