On Backstory

(I have to do homework soon, so I'm going to try to be direct in this).
You have a story, yes?  Your life, I mean.  You were born, then you did stuff, and now you're here.  Pretty simple, yes?
Naaaah.
Your story is what makes you - the little things that affect your choices later on.  That one time that you accidentally put lemonade mix in your hot chocolate and it tasted really good is the reason you now ALWAYS make your hot cocoa with a bit of lemonade mix.  (yes, this is somewhat biographical, and I do recommend it).  That other time you went rock climbing and pissed in your pants is the reason you now avoid rocks and the group of friends you went with.  Things like that.

Similarly, your characters have backstory.  They had moments where they were so scared they ran away and have never gotten over it, or they had such a bad experience with something else that they will go out of their way to avoid all things associated with it.  It is those things that make a person, wether or not they are part of a written story.
So give them a story before the story.  I didn't grow up in the same way my parents did.  Both of them moved around a lot, and the first time I spent more than three weeks away from home was when I went to college; other than that, I've lived in the same place my whole life.
But it's more than where they grew up.  It's the weird things, like lemonade in hot chocolate.  You don't have to put every little detail in there, obviously, but if you are going to make any character scared of anything, you need to give them a reason why, beyond 'they had a bad experience with it when they were younger.'  THIS IS THE TIME THAT YOU WRITE WHAT THAT BAD EXPERIENCE WAS.
With NaNoWriMo fast approaching, character writing becomes even more relevant.  The only excuse you have for not having a backstory is that your character is an amnesiac for 90 percent of the tale.  I'm still giving my amnesiac a backstory, though, because I still think it's an important thing to have.  It's a part of who she is and what she does, even if she doesn't know why she's so mad at the government or why she needs to find the King's advisor RIGHT NOW.

Though you don't have to include all the details (I wouldn't normally admit that I avoided bowtie pasta for years because I threw up after eating it one time when I was ten; I don't mention it because I'm not studying anything culinary and I don't intend do, and what sort of pasta I eat is not exceedingly important), include the bits that may be relevant.
And then throw in one or two "bowtie pasta incidents" for good measure.  They might be important.