Creating fictional characters is just as much about what you do away from the computer as what you do at the computer.
First, let's get one thing straight. When it comes to stories, characters rule. Above everything else. Plot. Setting. Pacing. Mood. Forget it, characters are what make a story and keep a reader turning pages. If you can convince a reader (hopefully) in the first chapter that the characters in the story are real people then you've got her hooked for the rest of the story. Readers want to know what happens to a character more than they want to know what happens in a plot. Readers want to be engaged, they want to feel connected to a character. They want to vicariously live that character's life.
If you can accomplish this as a writer than you've done your job. Well done. And the reader will keep coming back for more.
Second, what I do off the page has a direct correlation to how I create on the page. Meaning this: to craft believable characters on page I have to become a people watcher off page. I spend a lot of time studying my subjects (people). Wherever I go I observe. I take note of the way people dress, their gait, posture, body language, what they do with their hands. If they're conscious of others around them or in their own little world. I watch how they interact with others, listen to their conversations, pick up on their non-verbal communication.
Here's a fun article about microexpressions, or the subtle non-verbal hints someone gives to how they're truly feeling.
I work in and around Gettysburg, PA and in the summer there're lots of people milling around the town, the sidewalks, in and out of stores, restaurants, crossing streets, wandering the battlefields. I like to drive real slow through town and just watch humanity pass by. Every day is an education in human nature for me.
Also, and very importantly, when I interact with people I'm constantly paying attention to the subtleties of their behavior. Do they appear nervous? Are they distracted? Engaged? Bored? What do they do with their hands? How are they standing/sitting? I listen to their word choices, their vocabulary, grammar, sentence structure. I pick up on anything I can and file it away for later use.
Thirdly, I love talking to people, hearing their stories, their experiences, their opinions, anything and everything. The more you listen, the more you learn about that person, and more you learn the more you'll understand them as a total individual. I ask lots of questions and try to probe as deep as I can. I don't want to just know that a person is upset about the latest political news, I want to know why he is upset, how he thinks it affects him or his family.
Try it. The next time you're out in public really pay attention the people around you, watch them, study them, learn from them.