The examination of character

I guess I am developing a saying. Which is funny because I don't know if you develop it as much as someone else decides to attribute it to you. In this world of the internet, I find interest in how quickly conversations or posts online have been assessed with real conversations with actual people. I've had philosophical conversations which have been maligned in how I present things, even though a portion of which is silly because, I should be able to convey myself as a writer. People want to read anything into what you post, so here is my saying (developing of course):
"If your characters aren't good enough to star in a book by themselves, they aren't worth having even as a supporting cast."
Take that for what you will. I am starting to believe in it. That doesn't mean every character needs to be so colorful that they blind you from seeing the storyline or the main characters. However, they need to be focused enough to run their own lives. Instead of merely waiting for the main character to show up because the storyline calls for it. If you look at each character as their own person, their faults and assets, as well as their abilities and surrounding situations, and if they have enough "character" to exist by themselves, then they are worthy of being in a story.
What really makes a character? Is it that every character needs to have a special talent? No. But that doesn't hurt. Sometimes, it was the way that you convey how they grew up. If every character grows up the same, something is wrong. It makes the reader feel as if the mundane is the norm. That is not to say that they cannot do mundane things such as going to work, but something needs to be happening to show how they are different. Make them enjoyable too. That helps.
I base a lot of my characters on real people that I meet in a bar or restaurant. When people are commonly themselves, guard down, and there is little if anything to impress you on. Sometimes, other venues hold true to finding a character of a person. Because when someone doesn't need anything from you, their true character shines through. No bullshit from them allows you to see them, warts and all. That being said, it is best to be honest with the person in question. That doesn't mean telling them that you are observing them, but interact with the subject in order to see how they take life. Because no one is the same. If they were, it would be an awful waste of space.