1) Categorization
I pick MyersBriggs types for my characters, D&D Alignments, and I pick roles that they are playing at the beginning of the story.
For example, the 4 protagonists of my Urban Fantasy WIP are a team of drug dealers turned bank robbers: my first-person peripheral narrator is a Lawful Evil ESFP, his main character friend and boss is a Neutral Evil ISTJ, their mutual best friend is a Chaotic Evil ESFJ, and the ESFJ's younger brother is a True Neutral INFJ.
People worry that MyersBriggs doesn't work because only 16 boxes can't tell you everything about a person, they worry that Alignment is worse because there are only 9 boxes, and they feel that there would need to be 7.4 billion personality types in order to truly describe how a person is different from everybody else.
First of all, using both at the same time gives you 144 combinations to choose from (bit of an improvement I'd say
) ; second of all, even this combined system of 144 boxes doesn't tell you everything about a person, and it's not supposed to. A stethoscope is not useless because it only gives information about a person's heart rate and not also about their brain waves, rather a stethoscope is useful because it gives information about the person's heart rate.
2) Distinction
Once I have a baseline for my characters, I look at how they differ from other characters – both from my own work and other peoples' – of similar MyersBriggs/Alignment baselines (going back to the metaphor about the stethoscope: how are their brainwaves different from people with similar heart rates).
For example, my narrator is a Lawful Evil ESFP, but so too would I say are Darth Vader (Star Wars) and Don Maroni (Gotham). Darth Vader looks like an ESTJ at first glance because he's spent years keeping his inner personableness hidden under a veneer of professional military efficiency, but Don Maroni is very open about wanting to be seen as an ESFP people-person (unless you don't do your job). Darth Vader is willing to submit to the will of the Emperor, but Maroni cares so much about becoming the king of the hill that he ruins a truce with a dangerous rival by insisting on calling all the shots. Darth Vader is also a "law and order" Lawful Evil whereas Maroni is an "honor among thieves" Lawful Evil. My own narrator is as criminally-minded and as openly a people-person as Don Maroni, but is also willing to submit to my MC's leadership in the vein of Darth Vader rather than trying to run the show himself.
My MC herself is a Neutral Evil ISTJ like Dexter Morgan, but where Dexter only kills those who kill the innocent, my MC has no qualms about running violent drug operations that leave swaths of addiction and drive-by shootings in her wake.
3) Trial and error
If I come up with a bit of characterization that I decide later that I don't like as much as I thought I would, I see what I can do to change it.
My MC was originally supposed to be a Neutral Evil INTP who orchestrated bank robberies for the intellectual thrill of getting away with crimes, but then I came up with a back story about the group starting out as drug dealers, and I realized that she was more interesting as an ISTJ pragmatist who's just trying to make a living.