Some say the best way to write about characters is to study their faces when situations are presented to them. A good example of this is Dubliners by James Joyce, for he makes excellent observations on faces. Basic facial responses to situations may disclose much about a person.
In short, the way those that draw make studies of portraits with sketches may be analogous to a writer’s approach.
The next step is to bring that into both dialogue and intention, but this part requires depth both philosophically and psychologically, as one must ask why a character behaves this way or that. A more important question is asking what a character believes, and this is a gem brought down in Platonic dialogues. Characters in those dialogues are always pressed for beliefs, and one wonders why Plato did this, but this is better unraveled through examination in practice for your characters.
Plato is insightful with badgering for belief and Joyce for examining the face, and it would help to study them.
Not much depends on making oneself relative to this person or that in writing characters, for maybe then characters are limited by a focal point of beliefs, but on assessing beliefs and actions as a result of such beliefs. Do certain beliefs lead one this way or that? If not now, then will a belief lead this way or that? And this is where the philosophical element comes in, not one where one knows what Platonism or Aristotelianism is, but where the writer is hard pressed to explore another person with fundamental questions on love, life, death, and the metaphysical—this requires but curiosity and heart, not formal education, however, it doesn’t hurt to have such education.
This is similar to working with coworkers you do not get along with where, if you knew them, although both ultimately disagree on a fundamental value, you are pressed to ask if you would still have compassion for them: are you capable of such introspection? (The writer is not saying that you are not, but states the question out of necessity)
Such is the case for a writer, it seems.