One tool I've found useful in both RP character development and RL is the enneagram. I ran a search but didn't see it mentioned here before.

In brief, there are only nine primary personality types which determine how people see the world, interact, etc. We each also have a "wing" which colors our primary personality type. These have all been categorized and even have guidance as to how you'll change under stress.

For example, here is a description of my RL personality type, a "Four":

Characteristic role: The Individualist
Ego fixation: Melancholy
Holy idea: Origin
Basic Fear: Being commonplace
Basic Desire: To be unique and authentic
Temptation: To beat themselves up and withdraw
Vice/Passion: Envy
Virtue: Equanimity
Stress/Disintegration point: Two (Disintegrating Fours become dissatisfied like unhealthy Twos)
Security/Integration point: One (Self-actualized Fours are idealistic and progressive like healthy Ones)

More info can be found at the wiki entry. If it is something you are interested in learning more, let me know. Over the years I've gathered a number of books and can make some recommendations. One of my favorites is a book of cartoons that show an event happening, such as a car crash, and you are shown the viewpoint of the nine different types. Quite fun and very accurate. Given my personality type you can imagine my distress at being just a personality type 😀