Creating Conflict with a Crucible
It's human nature to avoid conflict. Certain characters (ordinarily bad guys) may seek conflict as a tactic for aggression, but most of your characters won't. In fact, even when confronted with conflict by a bad guy, it's often in your character's best interest to turn his back on the conflict and walk away. If the reader asks herself why your character doesn't just walk away, you're in trouble. Her suspension of disbelief is at stake. So, you need a device to believably keep your characters locked in conflict.
A powerful device for doing so is the crucible.
A crucible is a piece of laboratory equipment you probably remember from high school chemistry. It's a small dish with a pouring spout, a dish that can withstand extremely high temperatures. Chemists use a crucible to heat solids to such high temperatures. For example, you might melt sulfur in one over a Bunsen burner. Or you may combine different substances in one and meld them together in white hot heat.
In the vernacular, a crucible is any severe trial people undergo.
As a literary device, a crucible is a relationship imposed by place or situation that bonds antagonistic characters together while their conflict heats up. The situation may be emotional or physical — moral or material. Characters caught in a crucible won't declare a truce or quit or walk away. This is either because they are trapped or because the motivation to continue opposing each other is greater than the motivation to walk away. A crucible can hold characters in it for a scene or a series of scenes, but usually it holds them throughout the story.
A marriage, a jail cell, a family, a lifeboat, a foxhole, a military unit, and the workplace can be such a crucible. Intimate blood relationships are a crucible. Dependency is a crucible.