So this is going back about eighteen months, but I started a small series on settings in stories, and I realize that I never really finished it. So here is part three!
As I discussed in earlier posts, a person's life can be divided into three (not equal) parts: the work life, the social life, and the home life. Generally speaking, you can't focus on all three of them; one normally is pushed to the foreground and one is pushed to the background. But it is still necessary to know about these lives, and how a character acts differently in each one.
Perhaps the most interesting life to explore is the home life, because this is the base of a character. People always put on facades in other places; a person is supposed to act professional at work, and then overly-friendly around people. When we deal with other people, we change slightly. This gets a bit of a bad reputation, because it seems duplicitous, but in reality it isn't always. It is respectful to be polite and somewhat distant at work, while if we took that attitude into the social life it would be seen as rude. People have to change for their situation; it's their character that doesn't need to change.
But when you are at home, those pressures are gone. Generally speaking, you live with the people you are most comfortable with, or you have to become comfortable with those people because you all share a bathroom. You don't always have to be "on" at home. You can be who you want to be. As an introvert, that's what I love about being at home; extroverts might hate that. But it all goes down to knowing your character. Maybe your person is worried about upsetting people; when they are really at home, they don't have to worry about being polite and non-confrontational. They can be themselves.
As such, who a character is at home is generally a better representation of themselves. It is a good place to start when you are building a character because once you understand them there, you can understand them everywhere else. The tics come out, the quirks. Their actual thoughts and feelings. The character can breathe, can settle, and you can figure them out better.
Now, home life is also a fascinating aspect to remove--but even then, you need to know what it was like. Perhaps the best example is moving away to college. For the first time, you (or your character's) home life is replaced with more social life, but the home life will seep in eventually. You'll become at home with your roommates, or seek home life out somewhere else--but it has to come out eventually. All stories have to deal with what a character is like when nobody is around, and that's what the home life is about.
So figure out who your character is at home. Once that is set, layers can be added for the work and social lives, and then you can start to play.