Pay attention to how real people around you act.
I think this one speaks for itself, and we all know why. We cannot write impossible characters.
This is what I call the Superman effect. Superman is basically the perfect man. Strength, speed, and heroism up the yin-yang. He's always noble, he always does what's right. His only conflicts come from the fact that he's only one man and he has to save the sinking tanker at the same that a comet is hurtling to earth while he's trapped in a locked room with Lois Lane and can't reveal his secret identity. But we know he'll get out someway, usually by moving faster than the human eye can see (that old MacGuffin). Otherwise, he's never put into a moral quandray about whether it's his place to kill the bad guy (Batman Begins), to reconcile his human life with his superhero life (Spider-Man), control his power (The Incredible Hulk) (See: Superman is a Dick), or battle his own personal demons while fulfilling the need to help others (Wolverine). Superman is all-powerful and uncorruptible. Thus, he's boring. Superman no longer has a place in American folklore. He's made himself obsolete.
I seem to have lost track of my argument here. I think I was trying to say that all the above examples are humans, and Superman is not (well, he is an alien, but I don't think that's relevant because he resembles a human biologically). If you want to write characters, they have to have characteristics. Not necessarily flaws, but a real good character will have idosyncrasies.
Real people have obnoxious laughs, slow speech, certain gaits. They try too hard to be likable, interrupt other people, act spazzy. They either make stupid jokes, or don't say anything. They don't eat everything on their plate, or they're the friendliest people in the world, and you'd love to be like them and never can. They run marathons for breast cancer, and still live with their friends from college who now have a baby. They're too loud, they have candy jars by their desk, they carry too much weight, they look like Hardy from Laurel & Hardy. They're the nicest people in the world and are vegetarians. They struggle with balancing home, work, and life. They do tricks on wakeboards, and have cheesy mustaches.
Everything I've just said may seem fantastic or implausible, but they're all real characteristics from the people I work with. None of them are special, but they all have things that I can point to and say "That's what makes that guy that guy." And that's what you use when you make a character.