One is the loneliest number, but two makes a story! Put one thing next to a contrasting thing, and tell a tale. Juxtaposition: it is the meaningful placement of one element in close proximity to another element, and it is an essential aspect of successful, effective screenwriting that can range from plot, to characters, to setting.
The first way you can use juxtaposition is on the level of plot-mash together two plots and intercut from one to the other and back again. Every episode of *Lost* does this: having the Present Day Island A-storyline and the Flashback B-storyline revealing key background information about the Island story's protagonist. The A and B storylines loop back and forth on each other, commenting on the psychology and origin of the character (back when he or she lived on the mainland) as he or she navigates the mysterious island.
In *American Gangster*, Steve Zaillian juxtaposes two present-day narratives separately starring the two main characters on the opposite sides of the law. Only in the end do the cop (Russell Crowe) and the criminal (Denzel Washington) finally meet, and the startling resolution is achieved.
A second way to juxtapose is to place together two intrinsically contrary characters and witness what sparks soar. With Neil Simon's *The Odd Couple*, we find neat-freak, soft-spoken, soft-hearted Felix Ungar and his sloppy, brusque, burly best friend Oscar Madison at an identical place in their lives and having to share a single living space.
Undoubtedly, Shaksepeare was the greatest master of juxtaposing, and specifically characters. In *Othello*, the Moor General falls in love with the white, upper-class Desdemona, much to society's consternation-race inter-relations are dramatized to a heartbreaking result. With *Hamlet*, the story only takes off after the meeting of a living young prince with the ghost of his father. Here, Shakespeare juxtaposes inhabitants of this world with the next-grave ramifications inevitably follow.
Take any crime drama and you'll find the juxtaposition of two groups and ideologies: the law and the lawless. William Monahan's *The Departed* juxtaposes the Boston Mob with Boston Law Enforcement. With the unique case of William Costigan (Leonardo DiCaprio) and Collin Sullivan (Matt Damon), these guys are secretly members of the opposing tribe. Monahan switches between their worlds, showing the good guy Costigan seriously stressed out and fearing for his life while the villainous Sullivan struts around the office in his suit and tie, smiling, impressing his boss, and never too tired to flirt. This juxtaposition would strike the audience as profoundly unsettling if we didn't believe the bad guys would be held accountable for their sins and insidiousness by the bloody end.
More broadly, juxtaposition of groups can perform in any narrative where "bad guys" and "good guys" clash. Or even when, in a romantic comedy, boys meet girls. Now there are two groups that, whenever put together, are guaranteed miscommunication and radically divergent agendas.
The "fish out of water" paradigm epitomizes the juxtaposition of character and incongruous environment. In Karen McCullah Lutz and Kirsten Smith's *Legally Blonde*, cute-as-can-be Elle Woods (Reese Witherspoon) bops along through the Ivy halls of Harvard Law School. Surprising everyone who's so silly as to judge her, Elle proves that she's got quite the knack for rhetoric and logic-and have energy leftover to whip together a hot pink ensemble!
Lastly, you can juxtapose one setting and another setting. Larry McMurtry and Diana Ossana's *Brokeback Mountain* oscillates from one world (the pristine, simple paradise of Brokeback Mountain) to another world (the repressed, repressive labyrinth of the heterosexual modern American West). Up on Brokeback, away from their spouses, children, bosses, and invisible, murderous foes, Ennis (Heath Ledger) and Jack (Jake Gyllenhaal) are free to express their love and affection for each other-to be authentic. Once they go back down to civilization, however, they must resume their socially-constructed and necessitated masks of conventional cowboy masculinity. Jack wishes they could create a place like Brokeback to be together always, while Ennis is reluctant to leave behind his obligations and the rest of the world, miserable though he is.
In what ways do you use juxtaposition in your screenplays? Do you maximize your narrative's dramatic or comedic potential by placing a character or action in an unexpected environment? If you feature two parallel plotlines, do they necessarily interrelate and come to a cathartic synthesis by the conclusion? While "two" makes for story material and exciting conflict, you are still the "one" at the computer: it's all in your hands.