For me, I use these questions as a jumping-off point:
The 100 Most Important Things To Know About Your Character (revised). I like these because they range from basic questions to more in-depth questions. But character questionnaires aren't there to be filled out like tax forms. They're there to get you thinking.
The most important question I ask myself when "creating" a character is this: what matters to them, and how does it affect how they see the world? For example, my character, Card, often gets lost in his own thoughts, to the point where he completely fails to properly listen to the people around him. He's a deep thinker, but he doesn't know how to engage with the world--he's learned through past experience that the world is callous and cruel, and adults and kids his age aren't worth listening to. When he finally does find some people he thinks are worth listening to, he finds that he can't just flip a switch and pay attention now. By failing to see that everyone is important in some way, he's lost the ability to pay attention to what really matters.
The important thing about the character is that he changes over time. What matters to him changes over time. He learns. I think some of the most interesting characters are the ones who have good arcs.
But the key to writing interesting characters is also to write uninteresting ones. Not everyone can be at the forefront of the novel. That guy you're passing in the street? Most of the time, he's just a guy to you. Sure, he's a person with his own inner world, but because the guy doesn't matter to your character, their worldview, it doesn't matter to the story.
The reason I put "creating" a character in quotes is because honestly, I don't really believe I'm creating anything. The characters I'm supposedly creating feel far too real. I believe they're somewhere out there, in a parallel universe, somewhere in the ether, and I'm just listening to different versions of lives they've lived. When you take the time to actually listen to what the character you're creating has to say, I think you'll find they develop their own voice and personality quite easily, since their "creation" was never really yours to begin with.
