Imagine a world peopled entirely by yourself, only one has an eyepatch, one chews bubblegum all the time, and one speaks in a Southern accent. Scary, huh? Well, that's the world too many people end up in.
The key is to create unique, believable characters that are as different from yourself as your crazy brother is. And how do we do that? Character worksheets, character interviews, character templates...all of these are great tools for discovering motivations, back story, etc. But let's face it, without material, these things are worthless.
I think the best thing a writer can do for their characters is to go out and meet some. Observe people. Set up post in the mall or a park or on a bench downtown and just watch. Yeah, it may seem a little creepy at first, but get over it. Being a fiction writer is a little creepy. We create entire worlds in our heads and spend hundred of hours transcribing them onto paper. We meet people, fall in love, fall in hate, commit murders, have sex, pet dragons, get pregnant, dye our hair, and on and on...all in our imagination. And then we expect people to give a damn about any of it.
I, personally, revel in the slightly creepy. And the slightly more than slightly creepy, too, if we're being honest, here.
Anyway, have a seat, pull out your notebook, and start writing things down.
I think of these things as 'bright spots'. I'll give you a couple examples, straight from my own tiny notebook.
--A wall made of boulders- half as tall as a man, and twice as wide. What's hiding in there? Great big gaps, big enough for a newborn.
--Japanese assassin- sent to the shrine to off a man thought to have buried stolen treasure at the abandoned shrine. Finds something else instead.
--Angry girl behind the wheel of a yellow sports car, driving nowhere, fast.
And on and on it goes. I've got half a dozen of these little books filled up with things I will likely never look at again. The point of them was to internalize that moment, that idea, that character trait. By internalizing it, making it a part of me, I will have that at the ready when it is time to create a new character or story or setting or scene.
Experience is vitally important to a writer. Not necessarily experiencing all sorts of craziness like bungee jumping or riding in a spaceship. Just getting out there and soaking yourself in the bright, amazingness that is mankind, getting to know how people work and think, is crucial. Without that context, everything else is moot.