No one wants to spend their time reading characters that aren't believable. We have enough people who can't hold our interest in real life—writing them into your stories is a disservice to a faithful audience. Making characters three dimensional takes planning and consideration, and even if you're not careful, they can slide back into flat-dialogue-speaking-feelingless stick figures.
So how do you write real characters into existence?
#) Study people. You'll understand how to make the "fake" thing if you have complete knowledge of the real thing. Watch people interact, watch them be alone. Study their mannerisms, their habits, their hobbies. And ask questions. Understanding the why behind the what always helps to write more whats.
#) Make friends. If only for the sake of your writing, make friends with your characters, even the villains. In real life, you're honest with your friends, and you see their flaws. Do it in your writing, so you can give them believable flaws (nobody really likes sheer perfection) and lovable foibles.
#) Let them go. One of the delightful things about creating characters is that once you've given them life and personality, they'll start making their own decisions. When they do that, don't try to force them back into the mold you've created for them. Let them do their own thing, and when they suffer for bad decisions, don't try to patch it up for them right away. Let them be real people, who mess up and get mustard on their clothes and sing off-key.
#) Practice. I include it in every list because it's really the most important thing to do in writing. Stop reading this and go write up some real people.
We like to read us about people who remind us of ourselves—if you can master creating them, you'll be miles ahead of all competition.