As a kid, I was charmed by Stephen Vincent Benét's short story, The Curfew Tolls. It's a gentle alternate history tale the features an epic historical figure who does not achieve greatness because he is born decades too early. The conceit is circumstances are critical to revealing the qualities of a hero or heroine.
I loved this story and the idea of putting characters in different situations that challenge them. (I probably am more interested than most since my father liked to explore this notion. He told me it was a key idea in his masters thesis.) It has provoked me, at times to drop my characters into scenes that have nothing to do with the work in progress, just to see what would happen. Sometimes, it amounts to cruelty. (Good thing your characters can't sue you.) Sometimes, it helps me see them in new ways that become essential to enhancing my novels. And, I'll admit, there are times when versions of these stories become short stories where characters get a second life.
For you, if you are a novelist, screenwriter, or playwright, it may provide a way to explore characters and get to know them in important ways. So I'll offer five situations that you can use to test your heroes, villains, and sidekicks.
Life or death. Put your character at extreme physical risk. I like to dump them into real instances where (rightly or wrongly) I feared for my life, but it's okay to imagine something that is vivid. You may even grab a scene from a war movie or a science fiction thriller, but don't try to sell what results. That's called fan fiction and can cause copyright problems.
Humiliation. You can start with a look at your own life, but don't stop there. What embarrasses you might not embarrass your character, so keep that in mind with this one. Essentially, humiliation is about being victimized by having a vulnerability or fault revealed publicly. This one might make you squirm as you write it. Good.
Last chance at love. Here the character must choose to reveal his or her heart, and it must represent accepting a difficult change, facing a fear, or opening up to life-altering risk. And it must be the last opportunity to do so. I like imagining grand gestures, but a romantic comedy run to catch a departing love can work, too. And sometimes, it's just an answer to a question or a declaration. Whatever you choose, see if you can crank up the stakes in some way. Declaring love in public, especially in a hostile atmosphere beats a private admission.
Temptation. For many characters, this is weighing getting what they want most against sacrificing integrity in an essential way. For characters without clear moral structures, it may be letting go of something foundational to self image. In the best of stories, this is what happens at the climax, with the character showing growth by choosing integrity over what he or she desires above all else. In Casablanca, Rich chooses striking a blow against the Nazis over his love for Ilsa.
Dilemma. Give your character an irrevocable choice between two damning options. Superheroes face these all the time. Defuse the bomb in the hospital or rescue your friend from the rampaging robot. Save yourself or deliver the plans that can disable the world-destroying weapon. Making it personal is an essential element, and you can double down on that. Sophie's Choice provides a good model.
For each of these, it's important to enter fully into the situation. Don't just come up with a one-sentence answer. I try to invest forty minutes in drafting a pivotal scene that takes full advantage of the implications of each situation. The prose does not need to be wonderful. The point is to confront the possibilities with imagination and honesty.
Afterward, do a post mortem and see what you've learned about your character and how that knowledge might enhance your real story.