My technique for creating believable characters
I mostly interact with people online, but I do go outside of myself with those that are in the real world as well. This is how I create a great deal of my characters.
I watch and pay attention to the appearance and mannerisms of people in my apartment building, my family and others that react with day to day. I will also watch people that walk by in the mall, or coffee shops that I patronize.
I see a particular quirk-like turn a of phrase that a person will use frequently- or a hair style that I find interesting and try to integrate it into a character. My characters' clothes are a combination of fashions that I dream up and fashions that I see the people around me wear. All of this adds up to an interesting character.
I adopted this method because I realized that making up most of my characters was not only tedious but only adds up to unrealistic characters. Resonating, believable characters resemble the people that readers' deal with everyday. Therefore, we have to integrate traits and habits of real people into our imaginary characters. They must be part real and part imaginary in order to jump out of the page for the reader.
Your routine of character construction might be different. I would encourage writers to try this technique. I does work for me. It might work for you.