Applied Physics

Despite my deep loathing of the subject, I've found that physics can, in fact, be useful. Even to writing. More specifically, I found the explanation of elasticity to be quite relatable to how I tend to develop my characters. Here's a lovely illustration for those unfamiliar with this concept:

[image from http://www.tutorvista.com/content/physics/physics-iii/solids-and-fluids/elasticity-modulus.php]

At the beginning, the stress (basically, force or pressure) gets added to the object, and the strain (stretching or compression) of the object increases proportionally. So, when the story just starts out and the character's troubles have only begun, he can adjust to the stress put on him and grow without being harmed too much.

Once the elastic limit is hit, things start to get weird. No longer is the relationship between stress and strain so simple. The object can still stretch and compress, but it doesn't take much more stress, and, once the stressor is removed, the object can't return to its original state; it is permanently deformed. So, once our main character reaches a certain amount of stress or trauma, it just isn't something he can adjust to. He can indeed rise above the circumstances, but he has to stretch himself too much to do so, and he'll just never be able to go back to the person he was.

Last but hardly least is the fracture or breaking point. What happens to the object there should be obvious. The character, likewise: whatever he used to be before, he is now but a broken shell of a man.

Exactly how much stress you want to put on a character, or how much he can handle before hitting one point or the other, can and ought to vary from author to author, story to story, and character to character. Want to break everybody? Go ahead. Just warp them? Sure. Break a few, warp a few, and just stretch the others a little? No problem. It just depends on what kind of story you want to tell and what kind of characters you want to deal with.