When I was still in the beta stages of Spirit of Orn, I had not yet developed a concrete method of building characters yet. It helps to have one, or at least guidelines.
The details that go into a character are important, such as what their profession is, or their particular mannerisms I've written about this before. When I was at the auto mechanic shop though a few weeks ago something dawned on me that has really changed the way I write characters.
Both this guy, and someone else that I used to work with (both mechanics) has this way about them. It's that look or attitude that conveys their confidence but it's overshadowed by inadequacy. It's in the way they talk. "I'm going to tell you something that you need, but my voice has a slight panic in it. Did I really make the right decisions? I'm a fucking mechanic! People I worked with in High School are making big money now because of their ambition and decision making. What am I doing working on Honda Civics in a crappy part of town?"
And this can work for anyone.
You need to ask, very critically, "Why do you do the work you do?"
What makes a lawyer a lawyer? Imagine someone who in school who strove to achieve his goals. Were his parents distant? Did he do it to get recognition or fame? Did he ask questions like, "How hard do I need to work until you acknowledge me? Look at all that I've done! Are you proud of me yet? Do you love me?"
These are core needs and desires every character possesses.
In Spirit of Orn, as I work on my third draft I'm trying to do this. For instance one of my characters was a wild child, abandoned at a young age by his parents, to be found by one of the supporting characters in the novel and raised by that person. A good friend of mine has adopted a child similar to this, and so when I went back to write this character I asked my friend more about his son and what his son acts like. I found out that wild children are very good at telling you what you want to hear, or are very good at smiling when they don't want to. It's all an act rooted in their need to survive. Back in his country of origin, this child would need to lie to get food, or even steal it, and even though my friend has done nothing but raise him in a supportive loving environment, it will take a lifetime to reprogram this child. The wall of distance and safety that is constantly raised will never be lowered, and nothing but good parenting and structure will defeat it.
So those are all good details, but how do you apply it to your book's characters?
Essentially this particular character I had in mind I needed to make emotionally distant. He could be charitable and helpful to the main character, but the wall needs to be there, and only after a traumatic moment does it truly go down, and you see how barren this person is underneath.
Ask these questions. Apply your answers. Do it creatively, and it will take you to good places!