The all-important crucible

Author and publisher Sol Stein says one of the keys to a good story is to put your characters in a crucible and see how they react. As I recall from my high school chemistry days, a crucible is a thing in which you put various ingredients, apply a flame beneath the vessel and make a record of what happens.
We all experience crucibles in our everyday lives. As writers, we must understand that there is always more than one person in the same crucible. How they react to the same circumstance is what makes the story.
The Titanic was a crucible.
Each of the lifeboats were crucibles.
In Hemingway's "The Old Man and the Sea," the boat and perhaps the sea are both crucibles.
In the "All in the Family" television show the Bunker household was the crucible. Think of how differently Archie, Edith, Gloria and "Meathead" all reacted to living under the same roof. Archie was the dominant figure. Edith was the doting wife. Did she resent her servitude or had she simply accepted it? "Meathead" and Archie frustrated one another but "Meathead" showed no signs of wanting to move out.
Life is full of crucibles. For some people, their marriage is their crucible. For others, it might be their job or a disabling illness or something on their conscience that dictates how they think and act.
Identify the crucible and treat it with respect.