Steven James has written a great article in which he explains putty people and pebble people:
There are two types of characters in every story—pebble people and putty people.
If you take a pebble and throw it against a wall, it’ll bounce off the wall unchanged. But if you throw a ball of putty against a wall hard enough, it will change shape.
Always in a story, your main character needs to be a putty person.
When you throw him into the crisis of the story, he is forever changed, and he will take whatever steps he can to try and solve his struggle—that is, to get back to his original shape (life before the crisis).
But he will fail.
Because he’ll always be a different shape at the end of the story than he was at the beginning. If he’s not, readers won’t be satisfied.
Putty people are altered.
Pebble people remain the same. They’re like set pieces. They appear onstage in the story, but they don’t change in essential ways as the story progresses. They’re the same at the ending as they were at the beginning.
And they are not very interesting.
This article also explains the 5 essential story ingredients.
Thanks to Steven James.