The Myers Briggs personality theory was a major help in the characterization of my OC's.
In the words of the Myers & Briggs Foundation- "The purpose of the Myers-Briggs Type Indicator® (MBTI®) personality inventory is to make the theory of psychological types described by C. G. Jung understandable and useful in people's lives. The essence of the theory is that much seemingly random variation in the behavior is actually quite orderly and consistent, being due to basic differences in the ways individuals prefer to use their perception and judgment".
It bases it's result on these key personality traits-
Social
(E) Extroversion - thriving around others
(I) Introversion - thriving alone
Information Processing
(S) Sensing - pays more attention to the five senses
(N) Intuition - pays more attention to impressions and patterns in the environment
Decision Making
(T) Thinking - based on logic and consistency
(F) Feeling - based on people and circumstances
Structure
(J) Judging - clean cut, schedule based
(P) Prospecting - open minded and slightly sporadic
It can be hard to fully flesh out a three dimensional character. There are questions an author needs to ask themselves; how would they react in this situation? Are they more of an introvert or extrovert? Would they rather spend the day in a quiet room with books or at a bustling bar with loud music?
By answering these questions, It makes the character come to life as a real person. The Myers-Briggs test helps to make these decisions. By taking the test and profiling my character, I saw what needed to be added and further developed about him to have him appear as a real person.
This means he is warm and unassuming in his own steady way. He's efficient and responsible, giving careful attention to practical details in his daily life.