From a few carefully chosen beginnings, the characters just grow for me and I find I'm no longer creating them, I'm getting to know them. The first thing I want to know about a main character is, what is this character's passion? Then I want to know what is the one thing that makes this character different from the average kid on the street. And, from my experience in theatre, I want to know what the character wants and what his motivations are. If I am playing a part on stage or directing a play, I need to find things that can actually be used to show the character. It's nice to know a lot of little character details that may or may not work their way into the story, but as one of my favorite directors likes to say, "Yes, but is it playable?" Can I actually use that detail to build the character?
If it's a supporting character I need to know what makes him similar to and different from the main characters who the story is about. The character quirks seem to grow out of that. After I had finished my current WIP, I found there was a scene in which I needed a certain character to say something very important to the MC and there was no one in my cast with the right attitude for this important speech. I created a new character an incorporated him into the story, starting with that key scene. The character was built around creating a believable character who would deliver those lines; a boy with a secret who had just moved to town from Queens. His different perspective was just what the story needed. I also take into account things like birth order. How are my characters' personalities developed by their interfamily relationships? Is this character under the domination of an older sibling?
I also want to know little signature quirks. My MC is constantly wearing his favorite T-shirt, a shirt with a graphic that is very telling about his character. One of my girls is quite fond of saying "oh, crap."
And after 32 years as a teacher, I must admit that I have stolen seeds of characters from many kids I have known through the years.
Another peculiarity for me is that for adult characters, I want to know what kind of car they drive. That tells me pretty much everything I need to know about that person. For example, the mother of my current MC is a school librarian who drives a purple PT Cruiser. The principal of the MC's school is a woman who drives a black Corvette convertible. Once I know what cars my adult characters drive, everything falls into place as far as character development. It may not be a "playable" detail, but playable details can grow out of this.