My cousin once used the term "default emotion" to describe how people react under pressure.
Stress someone a bit and their default emotion starts to show. Perhaps they become aggravated. Perhaps morose. Perhaps fretful. Perhaps they become inhumanly efficient and focused.
Characters in art also reveal their true selves under stress. You will know the hero's fine character by how he conducts himself in a bad situation, and you will know the villain to be a jerk by the same.
A plot is all about stressing your protagonist. You give them a problem, let the problem snowball out of control, and then document all the sad, desperate flailing your main character does as they leap from one attempted solution to another.
Your protagonist's qualities should be revealed by their flailing. If they're intelligent, they should come up with clever solutions, if they're courageous, they should do something brave, etc. The resolution of the story's plot should be something only the protagonist - with all their personality quirks - was equipped to bring about.
Which gives rise to an interesting dilemma - do you come up with the character first, and let their personality dictate what the story's resolution has to be, or do you come up with the climactic scene first, and let it dictate what sort of person your protagonist must be?
Which method do you prefer to use, and why?