Good and evil: Star Wars. Black and white: The Defiant Ones. Men and women: When Harry Met Sally. These films are based on polarity, with characters so diametrically different that conflict is inevitable.
Sometimes the polarity is a cut and dry opposition between protagonist and antagonist, but often it occurs within an unlikely team: Lethal Weapon, Shanghai Noon, The Odd Couple. A classic example is The Defiant Ones: Two escaped convicts chained together, one white (Tony Curtis) and the other black (Sidney Poitier), must overcome their prejudices and work together to elude capture.
We need polarity in screenwriting, not only because it creates great conflict, but it also allows for the opportunity for a character to change his or her position on race, sex, or ideology. Change is strength, and if we watch a character start seeing the world differently – with an open mind – then there’s hope for the audience as well.