Characterization is a misunderstood task in my opinion.
Often when I am creating a new character or personality in a piece of short fiction, or even constructing the mind of a non-fiction character from historical sources, my first knee jerk reaction is to start with a concept, or maybe an over-arching symbol that the character will represent. This interprets the character and his/her actions as the sum of an allegory. I tend to start here, at many times on accident, or even by mistake, but there is a "better way" I have discovered that you all might find beneficial to your character.
Characters are people, just like us. They possess feelings, emotions, and a capacity to will and decide, and it is our jobs as authors to step back and stop controlling them. Though this is a bit abstract, if this is where we start, the product is always favorable because at least now we have stopped thinking of characters as our creations, or intellectual property. Generally to facilitate this organic process, we must start at the soul of the character.
Every character must be human. This is advice I gleaned from an interview with Alan Moore that was conducted a few year back (where I read it, I've since forgotten). He said that as outrageous and fantastic a character might appear, as long as they possess a human like feelings and emotions they will be identifiable. Characters that we can relate with are the best for the reader, especially from a commercially viable standpoint, because it gives them something to grasp and hold on to.
To do this we start at three dimensions every character must have: motivation, profession, synergy.
A character's Motivation is replete with actions. Actions/behaviors, which are the observable behaviors of the character, construct agency and personality. If you consider how you act and move in this world, the only things you do are things that you find meaningful or consider important enough to act upon. Actions also help the reader participate in the story's narrative helping them to construct in their minds who the character is through their mannerisms and behaviors. Usually they will don their personal experiences to make this possible.
Often overlooked, a character's Profession is vastly important to building a character because in many ways as I am sure you are aware of, we are what we do. Even more importantly, we are what we don't do or even what we desire to do but can't in the face of circumstance. This will help build up realistic characters and help to facilitate how the character sees and expresses himself in the world. If your character is a train operator for instance, he is a man/woman well traveled but affixed to their path, unable to deviate. They will be a slave to the routine and the mundane, etc.
Last, but certainty not least is Synergy. A character must interact with others to exist. I stress this because how a character reacts to his/her friends, co-workers, foes, and relatives, constructs their identity into a relational, multifaceted individual. Without this individuality, your character will be one dimensional. Now I am well aware of characters that are solitary in literature, like in Kafka's Metamorphoses, but be aware of what the author is doing. In this particular story, the lone protagonist is not set apart but rather courting solitude. I consider it to be playing with negative space. Your character in solitary confinement synergies with the lack of a companion.
And there you have it! I attempted to make this brief and to the point, but I hope it was helpful in your writing journey.