Making Memorable Characters

   What is it that makes you remember a person years after you met them? Why do you still remember the name of the kid who bullied you in middle school, when you can't even remember the hair color of the last person you passed on the street? The answer is: The person you passed on the street wasn't memorable. You remember people because something they were, something they did, or something that happened to them, was memorable. It didn't have to be big. Maybe another shopper spoke to you when most wouldn't even make eye contact for more than a split second. Maybe a customer went out of her way to make you smile. Maybe they themselves weren't extraordinary, just a situation you saw them in or were in with them. Do you still remember the guy you spilled gravy on in high-school? What about the kid in first grade who broke his arm while you were watching?
   Basically three things make a person memorable: either there is something extraordinary about them, or they did something extraordinary, or something extraordinary happened to them. Very often it is a combination of the three. This brings me to a rather obvious point:
   Extraordinary books need extraordinary characters.
   Think about it. Out of all the books you can remember reading, which ones did you enjoy the most, and why? There's a variety of possible answers, but chances are, your favorite books had colorful, unusual characters, even if they weren't specifically what made you enjoy it.
   So how do you make Your characters both memorable and realistic?
   Here's some suggestions I've come across in my own search for reality:

1. Pattern them after someone. My favorite character is patterned partly after myself, partly after the person I would like to be. Sherlock Holmes was patterned after a professor of Arthur Conan Doyle. If you use this, don't pattern them after someone close to you, and disguise it carefully. You don't want to be sued or have a friendship ruined because someone recognized themselves and/or didn't care for the way they were portrayed.
2. Talk to your characters. That sounds a little weird, but once you've got a basic idea of what you want them to be like, figure out what they would say and how they would act in response to certain questions and situations. You don't have to do it out loud, just talk back and forth in your head if you have to.
3. Give them their own personal quirks. Every person you meet has something weird about them, some ridiculous little mannerism or ritual or way of saying a particular word. Giving a character a dash of weirdness makes them seem more human. Be careful not to overdo it. To much weirdness makes a person, well, weird, and unless that's your intention (E.G. the mad scientist type) don't add too much.
4. Don't make your people Black and White. A trap many authors, even the bestsellers, fall into. The perfect hero, thwarted by the perfectly bad villain. The Superhero vs. Archenemy stereotype. The truth is, we are all fallen. We all make mistakes, including those who only exist in the minds of the author and readers. It's the same way with villains. Every person, no matter how bad they are, had a time when they were not. Every person has vulnerabilities. Giving them a past, an incident in their life that made them choose to go the way they did, will help, even if it's known only to you.

So what do you do to make your characters realistic?