How do I make my characters believable?
I based them off of real people. Not entirely, but the people around me have little quirks - without naming anyone, I'm surrounded by gum-smackers, feet draggers, knuckle tappers, flip flop slappers....the list goes on. They also do some pretty cool things...none of them have the same vocabulary. Even if they are the same age, siblings, "besties" ... Everyone talks differently. Some drop g's, some swear every other word, others make up their own words, or have accents. The people around me shove, hug with one arm, punch shoulders, flip coffee tables when their team loses... I'm a fairly detail oriented person (though the rate I forget things you'd never know). I pay attention to the little looks, the body language, and the interactions.
I also think about everything, even in my day to day life. I think about how I would react to something or maybe how my best friend would react, or brother, or .... you get the hint. Sometimes it makes writing slower, but I loathe rewriting anymore, so slower works for me if it saves me rewriting an entire novel later on.
Each character I create IS their own person. They were each built differently and uniquely. Some have visible scars, others internal. Some say y'all and other say y'ins (that's Pittsburghese by the way). Some snort when they laugh and others rarely laugh. They have different fears, just like you and me.
No one is perfect, so a character shouldn't be.
Become a people watcher, have a camera just in case, and always alwaysalwaysalways have something with you to take notes.
Just my opinion. :)