Juxtaposition is defined as an act or instance of placing close together or side by side, especially for comparison or contrast. Juxtaposed characters are not seen very much in short fiction, but if you are writing a novel you will want to strategically design characters who are foils for your heroes to shine a light on the traits of those heroes. One of the best examples of the use of this device is found in Arthur Miller's play The Crucible. If you haven't read it or seen it (first of all that is a shame) it is a play about the semi-fictional events surrounding the famous Salem witch trials of 1692. In the play, John Proctor has a brief fling with Abigail Williams who then goes into the woods with other girls her age to "drink a charm to kill Goody Proctor", John's more than forgiving wife. In the play, the uneducated, quietly faithful, guilty but penitent John Proctor is given the foil of Reverend Hale who is educated, a fiery preacher and as innocent as driven snow. The opposite nature of these two characters brings out the character traits of John Proctor, helping us to understand John's motivation, heroic yet tragic traits and despair when he is led to the gallows.
What is needed in writing today are more foils like that. I have intentionally created a few foils for my main characters in my current novel. Here are a few guidelines:
1. Make a List - Make a list of your main character's traits. Look at the character bio you created or their mind map and then start listing opposites for those characteristics. Create a character based on these opposite qualities and then use them in the novel. They do not have to be a villain, but they might be a grumbling anti-hero or a good hearted criminal.
2. Don't Get Too Serious - Even though Reverend Hale is a major character in the play, he is not as important as the main characters like Abigail Williams and Elizabeth Proctor and not as "fleshed out". Make your foil someone who is noticeable but only enough to bring to light the character traits of the person you are trying to emphasize.
3. The Best of Friends - Sometimes the foil in your novel is like Mercutio is for Romeo. Shakespeare was able to create foils that were not only friends of the hero but lovable and memorable characters. You might create a foil that is not a villain but a companion or a sidekick. Just make sure you make them opposite enough without overdoing it or the sidekick will overshadow the hero.
Above all, think about your narrative as more than a plot. It is not just important to tell a good story and keep readers entertained, but it is even more important to add nuances to your novel that are seen by a trained eye. The casual reader will find it that much more rich.