In everyday life, a crucible is a container used to hold metal or liquid as it boils. For example, to melt gold, one takes a heavy bowl made from steel and sets it in a fire. The steel, which can withstand higher temperatures than gold, doesn't melt. But the small container quickly becomes super-heated, so that the gold liquefies in moments.
In fiction, a crucible is any setting, condition, or relationship that keeps characters (such as a protagonist and an antagonist) from splitting apart.
By forcing these characters to stay together, we may sometimes create an almost intolerable atmosphere. It allows us to super-charge the relationships, raise the heat.
For example, imagine that John and Mary have been married for years, but have grown apart. They decide that they don't love each other anymore. The logical thing for them to do would be to divorce and split up, right?
But there's no story in that! The characters could easily resolve the situation by leaving--so as a writer you need them to stay together.
So imagine that John and Mary have grown apart, but both love their six-month-old daughter. Neither is willing to end the relationship so long as they risk losing the child. Now you have a crucible, a binding force that keeps the two together.
But there are different kinds of crucibles. Maybe it is a child. But maybe you could do the same by putting them both in a car and having them get stuck in a snowstorm. The car is a different kind of container from the relationship, but both work to keep the couple together.
So here are the three different types of crucibles.
Crucibles of Setting—A setting may act as a crucible. You've all seen comedies where several people are stuck in a cabin in a snowstorm, and each of them is at the other's throat. You will also quickly remember stories about people on airplanes or trains together. A crucible of setting might be a story set in your characters' workplace, on a ship, or in a small town. In Lord of the Rings, those who had joined the Fellowship were thrust into a crucible--a small band of men forced to band together for their own protection.
The important point is to keep the characters together as much as possible, and to let personalities rub against one another until their tempers boil.
Crucibles of Relationship—You can never escape your family. You might try, but often the family relationship is a crucible. A child wanting to leave home is in a crucible in the same way that a father who must pay child-support is in a crucible. Any two people who are married are in a crucible, as are any two people who happen to just be in love.
I recall a fine western when I was young about two heroic cowboys who are both in love with the same woman. They are forced to band together to rescue her from a kidnapper. The men hate each other, and as the audience gets to know each man better, they both come to vie for our affections.
Soldiers in a squadron will find themselves in a crucible. It may be that you find yourself fighting beside someone you detest--a murderer or a rapist--and yet you are unable to just walk away from the conflict.
A crucible may also be your conflict with your culture. We've probably all known various folks--Catholics, Jews, Muslims, etc., who try to leave their religion behind but can never stop talking about it. But it doesn't have to be your religious culture. My father ran away from the Blue Ridge Mountains to escape the hill-billy lifestyle. I had a girlfriend who left her fine home in Southern California because she despised her family's wealth. In the movie My Big, Fat Greek Wedding, we have a girl whose main conflict comes about when she is embarrassed by her ethnic roots.
Crucibles of Condition—An intolerable condition may also be a crucible—such as an illness that two very different characters may join forces to beat. We see this type of crucible used as Doctor House tries to solve the latest medical mystery. But you can also set your characters up to fight an economic or political condition--the hunger in India, the tribalism of North Africa.
The condition might be something as mundane as crime in the streets. Policemen who despise one another are often found joining forces to fight drug lords, rapists, and other types of crime.
So as you form your story, consider how you might strengthen your conflicts by developing one or more crucibles.
This week I'm teaching a writing workshop. I have another one coming up next month. You might want to check it out.