The Achilles heel

I may have posted about this before because it's a basic belief for me, but I've been thinking about it all day, so here it is:

Test a character at his/her most vulnerable spot.

If the character is an egomaniac, have someone puncture his vanity. If she likes to play the field and keep things light, have her develop intense feelings for one person. If he likes to keep to himself, put him in a situation where he has to deal with people. If she's impatient, put her in a place where patience is the...