The best way to learn about somebody is conversation with them. (Well, the best way is to hire a private investigator, but this is the best way that doesn't make you a super creep.) But in a regular pleasant conversation, you only get a small glimpse of who somebody is. You see pleasant things, funny things, enjoyable things. These are certainly real and a part of a person, but they are only part of that person. You can still learn more about the other parts.
I tend to talk with people about their problems. Part of the reason why is that I like to help people who want help. The other part is that I like to learn about them.
In order to understand somebody's problems, you have to understand their life. Who are the people in their lives? What kind of relationships do these other people have? How do they feel about each other? What actions and interactions are going on among them? What beliefs and philosophies do they have that makes them look at something as a problem?
The complex relationships and outlooks that we all have are part of what make us 3-dimensional characters. If you want your created characters to be similarly real, they will need to have their own people and concerns in life. Try learning about your characters from their problems. Once you try to solve that person's problems, you're halfway done with your story.