Think of the main relationship in the story as another character. Like a protagonist, the relationship can be tried and tested. Maintaining a positive bond can twist characters as much as obstacles and tortures faced in pursuit of a goal. And the most interesting results are likely to come when pressure is applied to a flaw.
Individuals’ flaws are well illustrated by the Seven Deadly Sins. If you protagonist is vulnerable to Lust, Pride, Sloth, Gluttony, Envy, Wrath, or Greed, he or she has a serious flaw that can expose character and create a powerful arc in the story. But what’s a relationship flaw?
Though the flaws themselves may be rooted in the characters, it’s valuable to tease out whatever might seriously threaten the relationship if tested. This is not the full set, but it’s a good starting point for storytellers:
Secrets - These hide parts of a character that are essential but problematic. Look at almost any romantic comedy, and you’ll see the main plot revolves around a secret the audience (or reader) is well aware of but the protagonist struggles to hide.
Insecurity - When one character feels unworthy of the relationship, it can eat away at the bond. If someone is eternally looking for signs of rejection, they’ll be found (rightly or wrongly) and the response is likely to be disastrous.
Wounds - People come into relationships with baggage. Often, betrayal, rejection, and even physical harm from another relationship creates associations that can lead to over-the-top reactions that may put a current relationship into a death spiral. If an abusive partner called a woman “Angel,” the new partner better not use that endearment.
Control - Part of the fun of The Odd Couple is Felix’s need for order and how it is countered by Oscar’s need for autonomy. Outside forces (a dominating boss, the death of a parent, the appearance of an old boyfriend) can amplify needs for control and autonomy to the point where they tear apart connections between people.
Competition - Sometimes friends egg each other on, leading both to achieve more. That’s fine. At the other extreme, if competitiveness leads to one person in the relationship sabotaging the other, that’s a betrayal. Jealousy and ego are usually at the heart of a toxic attitude toward competition.
Differences in values - Friends and even lovers don’t need to have identical value systems, but, without enough commonality, the depth of the relationship is limited. It also matters which values are unaligned. When bit differences in values around money, work, and fidelity, the relationship is in trouble. If the difference in values — including how promises and honesty are viewed — destroys trust, it probably means the friendship is over.
Communication problems - Listening is probably a more common problem that speaking. When someone doesn’t hear or misunderstands. Miscommunication is at the heart of many of the funniest farces and most heartbreaking tragedies. But communications can extend beyond words. Empathy is essential for a healthy relationship, so whatever event reveals a character lacks empathy for a friend or lover creates insights and excruciating choices.
Of course, a character inclined toward any of the Seven Deadly Sins can jeopardize a relationship. A guy with a wandering eye may be unfaithful to his girl. That’s Lust. If seven pieces of silver induced Judas to betray Jesus, that’s an example of Greed messing up a relationship. Or an honor that increases one character’s status can make him or her wonder why he or she is hanging around a friend or lover who is of lower status. That’s Pride.
This last points to a powerful tool for putting pressure on a relationships. Change. It doesn’t matter if its good fortune or bad, change forces a reevaluation and renewal of a relationship. The birth of a child is one of the biggest stressors in a marriage. The death of a child is likely to end the relationship. Winning the lottery or going bankrupt — both force partners in a relationship (whether love or friendship) to deal with hidden flaws that can break the bond. A declaration of love can be as unsettling to a relationship as a “we’re just good friends” comment. Big changes force people to look at past assumptions and update rules and boundaries.
Putting pressure in relationships is wonderful for stories, but difficult to do. An empathic writer will suffer along with the characters, and it’s tempting to avoid or calm the troubled waters. After all, almost all these injuries to relationships are agonizing in real life. Dealing with them honestly in a story can trigger horrible memories. So, even though taking on this challenge is a great way to make a story more powerful, it may not be worth it for you.
But if you can take it and you have the courage to probe these wounds, it will deepen your story. And there are two points of good news. First, you might learn something that helps you as a person as well as a writer. Second, in fiction, you can transform the trial into a happy ending.