Depending on the tools you use (in this post I'll be using MBTI/16 Personalities, which is pseudo-scientific but still a way we can measure), personalities are generally measured on levels of extroversion, abstract thought, rationale vs morals, and structured vs spontaneousness. Nobody is 100% anything, and the same is true of character personalities. While if a website asks you specific questions on how you react to a specific situation to get a general idea of what your personality type might be, a far more effective motive is to figure out what cognitive functions permeate the mind.
When Carl Jung studied personalities, he determined them based on these cognitive functions. There are 8 functions, and they are based upon introversion/extroversion of the function and the sensing vs intuition of the function. I.E: Ni = Introverted Intuition ('N' is the MBTI classification for intuitive, 'I' is introverted), Se = Extroverted sensing, Ti = Introverted thinking, Fe = Extroverted feeling, etc. You get the idea. Ti is internal logic, the ability to problem-solve, analyze, and it's bottom-up logic, whereas Te is related to goals, effectiveness, task-orientation, and getting work itself done. On the flip side, Fi is what feels right based on individualistic ethics and values, whereas Fe is social harmony. Ni is understanding the overall gist of a concept, the larger picture, and also understanding a wide range of concepts, whereas Ne is reading between the lines and seeing the context of a specific situation. Finally, Si is detail-oriented, memorizing, and mastering one thing, whereas Se is seeing things exactly as they are for their sensing.
That was a lot, but this is an oversimplified version! Each individual has these cognitive functions and characteristics. The question isn't a matter of whether we have these functions, but how strong they are. MBTI states that the strength of these cognitive functions determines our personality type. If your primary cognitive function is Ti, chances are your personality type is going to be an ISTP or INTP, whereas primary Ni says you're an INTJ or INFJ.
So why does this matter with characterization?
These personality tests exist for a reason. Again, while pseudoscientific, they have other meanings. The most general of these is to 'better understand' ourselves, but you can make the argument that as far as relationships and career paths go, they're useful to get an opinion from. For example, most detectives in fiction could arguably follow the ISTJ personality type because of their ability to look at the individual details, piece everything together, and then ultimately create their distinction on 'whodunnit'. Some of the most entertaining lawyers in all of fiction are ENTPs because of their chaotic, spontaneous energy, but also their ability to generate ideas and talk on the fly. They're not bad lawyers either!
There's a concept of shadow-types; i.e. one type shares none of the primary functions with their opposite type. Interestingly, the shadow-types often get a rapport and, despite their differences, are often seen as the idealized type for the other person. To get a shadow type, it's really simple: you keep the center-two letters and invert the outer-two. INTJ shadow-type goes to ENTP; ISFJ goes to ESFP, etc. Shadow types are interesting in that their cognitive functions mirror each other. For an INTJ, their typical functions are Ni, Te, Fi, Se, whereas an ENTP is Ne, Ti, Fe, Si. All of this is pattern recognition, but it serves its purpose for our particular thought experiment.
All in all, the cognitive functions and personality types we should take with a grain of salt. It doesn't determine everything; far from it. It's human nature to be contradictory. When I create a character, I never think of their personality type when making them. Post-production on a page, I could type them, but if I wrote them well enough, then they shouldn't fit perfectly into any type. For beginner authors who are structured, MBTI can be a useful tool, but it should not determine every trait. Everyone expects a stoic, quiet character to always be that way, but in reality? What if they're around their friends or family that they're close to? The people who know them better are people whom they likely would ease up their stoicism to enjoy their time with their family and friends. You know, assuming you haven't done the edgy part of their backstory where everyone they know and love is dead and for some reason it's just them who survived.
I always write backstories for my characters because it helps to understand how they think once you establish what they've been through. Why they think a particular way, why they're motivated to do X, Y, Z, but you seldom ever have to actually explain this backstory to the reader. Mostly, the backstory is your notes for that specific character on how they go about life. It's easy to write what might be amiss with them, but you should also write about the joyful times they've had in their life. What do they like to do when they're not being the hero (or villain)? Unless you're Rey Skywalker, your personality and dialogue are not usually based solely upon being 'the good guy', the 'hero', just as if you're the villain, you also don't think that you're the bad guy.
I posted about villains back on post #5 many weeks ago, but to sum up a little of what I said over there, real-life villains don't think they're evil. Real villains believe they're benevolent, that their ideology and methods are justified for the world they're trying to build in their vision. They justify their evil as necessary to eradicate what they themselves believe to be evil. While these fit the bill of many historical dictators who were loved by their people and hated internationally at that time, a lot about modern evil is less about promoting their ideas as the public's, rather promoting their own general welfare and greed. Being greedy is evil, and that's been done hundreds of times before (and we see why today). Generally, the best villains are either self-absorbed, egomaniacal, and vanity driven or charismatic to the extent that they believe what they're doing is right for their people, and their love for their people comes at a significant cost to everyone else. Or both.
Villains, I think, are much harder but much more fun to write than heroes. You can do a lot more with a villain's personality and give them good traits that foil the protagonist and, depending on if their traits are redeemable, they can even join the protagonist later to challenge someone worse. Often, writers will use the stock villain of wanting to set the universe on fire, but there hasn't been a single person in history who's been truly known for that. Even cult leaders are all about their charisma, their people; making the broken feel wanted by their organization. The generic dark lord who somehow took over a military regiment after being bullied in their childhood and then taking over a continent, threatening the rest of the world, because they just aspire to be barbaric and evil isn't substance. After all, their plastic-cup policies would've caused hundreds of revolts within a week based on their lack of medical care, food, water, and shelter. But geopolitics is a different beast altogether (stay tuned!)
Based on hero, villain, or even antihero personalities, a string of events happened in their past to make them who they are today. I enjoy using the phrase, "The difference between a villain and a hero's past is that the hero didn't break. The villain did." In a sense, it's correct. Morality is a construct for good, and there's a reason evil is the anti-moral (because... it is). In the real world, it's much harder to be a good, rational, and reasonable person who a generally likeable hero protagonist might be. Even if it's not a physically tough time for the protagonist, it is mentally strenuous. The villain snaps; they were once good, but once so betrayed by the world they believed so much in, they shifted away from morality to benefit themselves. Years down this path based on the strings of events that spiraled out of control, they're now this completely unrecognizable figure.
Anyway, this post has gotten a little long, so I'll sum it up. Write your backstories. Even if you never put it in the final draft, it helps you portray your characters so much better and understand their personalities, struggles, and desires.
Until we meet again, stay cool, everyone.
Dustin