One of the things we all try to do when writing fiction is to make our characters well rounded.. we don't want our Main Character, (MC), to drag around some cardboard cutouts that he talks to on occasion.. we want people who stand up on their own, not only do they walk and talk, but they yell, argue, spit, drink, swear, and fight on occasion I'm sure.
So how do we accomplish this feat of creating believable make believe people? One way is through backstory.
Say you have an old man who's grumpy and crotchety. He's always yelling at the kids to get off his lawn. Pretty cliche, right? You can almost see the prototypical old man sitting on a chair, or maybe a porch swing, in front of an old house that's badly in need of repair and a new paint job. You can see his manicured lawn, in contrast to the house, it's well cared for.. yet also a little neglected in spots. Maybe the grass is burnt in a couple of places.. the path up to his door has cement that's cracked, and there's some spots where some grass is grown through the cracks. Maybe his mailbox leans a little and has some cobwebs on it because he doesn't get any mail, not even junk mail.
Put this guy in your story and he's.. meh.. slightly interesting, but not original by any means. He's crazy old man Jenkins from down the street who has inhabited hundreds of stories similar to yours.
BUT.. now throw in some backstory on old man Jenkins.
Pretty big spread in character's, no? Old Man Jenkins is just a side character in your story.. but suddenly he might be a hero to thousands, or a hidden killer.. suddenly this cliche' bit character has a life, a motivation, and is interesting. How does this new deeper character change your MC's interactions? Maybe not at all.. maybe quite a bit. Maybe he grows from bystander to confidant..or secondary villain.
In fact... I'm kind of interested in learning more about both of these guys actually.