Writing Tips: Living Characters

What do some of the best stories written have in common? I would hazard a guess and say that the best stories, the ones that become classics and live through the centuries, have characters that come alive for the readers. So it follows that writers probably want to know how to make their characters come alive.
Admittedly, this is a skill I myself am still struggling with. But I think I can identify some pointers.
First, living characters are complex. This doesn't mean they have to have deep issues, or have some huge buried secret. It just means that they behave a little differently in certain situations. For instance, the reason I found Aragorn so appealing as a character was the fact that, brave, noble and diplomatic as he was, he was still human, and had some flaws. In the book (they skipped this in the movie), he has some real issues with being asked to leave his weapons--particularly Anduril--at the door before he and the others enter Meduseld. In fact, he comes about a hair short of throwing a fit over it, and it takes some stern words from Gandalf before he obeys. Now, I don't believe in making a character so flawed they're impossible to like...but a few little flaw will make them believable.
Another thing that brings a character to life is a regional accent. I have a little trouble with this...not because I fail to give them that accent, but because I have trouble getting out of a Texan twang. I tend to make people talk as if they're from the Southwest even when they're not supposed to be. (This is actually because I have very little experience with the accents of certain different places, such as New York.) It's a little easier when I'm writing for places that don't exist; when that happens, I just give accents to the characters who are supposed to have one and don't bother with the ones who aren't. For clear examples of different accents and how they work, I recommend Brian Jacques' Redwall series.
Then there's the quirk. The quirk is a small character attribute that you might not necessarily expect in that character. My personal favorite is Mr. Tulip, a villain from Terry Pratchett's book The Truth. He's a hulking brute who has no problem with violence and uses censored curses ( __ing) in every sentence. But he is also an art connoisseur who appreciates beautiful paintings and can recognize rare instruments at a glance.
Passion also helps bring characters to life. The heroines of Joan Bauer's stories are good examples of this: whether it's journalism, waitressing, gardening or shoe retail, they throw themselves into it wholeheartedly, sometimes centering their lives around their love of it. These girls love what they do and want to do it well--maybe even do it best.
And finally, believable characters are willing to engage in banter. You don't need banter; but it's something that brings a story to life and makes characters instantly endearing. And note: banter is only banter when both or all are willing to engage in it. If only one person is doing it, they will come across as either mocking or annoying. However, a teasing back-and-forth exchange is a lot of fun to read--and in my case, at least, often becomes my favorite part of the book. Banter isn't easy to perfect, but the end result is worth it.