Backstory can be a hard thing to get right. Use too much and the reader is suffocated with irrelevant information. Use too little and the reader knows next to nothing about your character.
So how do you find the balance? There are various methods.
Method One: Imagine the character as someone that has just entered your life.
Imagine that you're at a bus stop and have started up a conversation with a stranger. It turns out that you have a lot of things in common and chances are that you will see each other a lot more in the future. At first you might learn a little bit about their family, where they're from and what they work as. It isn't until some time passes and you get closer that you start to learn what makes them tick and what they've experienced to get them to that point in their lives.
Now think of this in terms of your character and their relationship with the reader. At first you start to get to know them and, gradually, you start to learn their history and the reasons they act the way they do.
Method Two: Use the backstory as a way to build mystery
Tease the reader by giving them snippets. Maybe the character has a particularly strong reaction to something that seems inconsequential or they close up when another character tries to discuss a certain topic with them. Here's a quick example:
His fingers tapped a quick staccato on the old oak table as he tried to hold back his agitation. She was still talking about the anniversary. Breathing deeply, his drumming sped up. His attention was wavering as she continued to ramble, his thoughts becoming dark as he thought of the last time he'd seen the couple- of the last time he's seen them without their terrible secret weighing heavily on his shoulders...
"Rick, are you even listening to me? I said that they're expecting us in half an hour- we need to get going."
His hand stopped abruptly, causing her to flinch slightly at the harsh sound of his palm slapping against the table. "We're not going."
"What do you mean we're not going- of course we're,"
His tone was sharp, his voice low. "I said we're not going. There's nothing else to discuss, Kelsa!"
Without another word, he left. The door slammed behind him with a resounding bang.
We don't know why he refuses to go, we don't know what their 'secret' is but by giving the reader that information we are giving them just a enough backstory to know that something is happening 'behind the scenes' and to keep their interest.
Method Three: Conversation
Now, this is quite a difficult one to get right. It is easy to make conversations sound unrealistic and difficult to process by trying to stuff it with as much information as possible. Often, in this case, less is more. Here's an example:
"What's gotten into you lately?"
"What do you mean?"
She sighed loudly as she crossed her legs, placing the magazine on the arm of the sofa next to her. "I mean that every time I try to talk to you about Jake, you close up."
Donna stiffened before forcing a tense shoulder to shrug. "There's nothing to say."
"Funnily enough, I don't believe you."
Feeling the sharp gaze of the other woman on the back of her head, she swallowed thickly and forced herself to concentrate on the CD collection in front of her. "You're imagining things."
"Damn it, Donna- will you just tell me! He's been asking after you everyday for the past two weeks and, quite frankly, I'm running out of reasons why you won't get back in touch. I think that after all of the lying and covering up I've been doing for you I have the right to know why you haven't spoken to this guy in the last five years."
Memories of a distant yell and blood stained carpets flickered through her mind as she tried to focus on the albums on the shelf in front of her. The reality of what they did, of what she did still echoed through her mind. Her voice was no higher than a whisper. "I never meant for him to get hurt."
Clearing her throat, she spoke louder. "No... I mean his uncle."
"What are you talking about? What's his uncle got to do with anything?"
Biting her lip, she risked a glance over her shoulder at the red head. "Five years ago, there was this... accident. A lot of people got hurt because of what we did."
A furrow emerged between her brows as she pulled herself up on the sofa. "Wait, accident? What kind of accident?"
"We got involved in something that we shouldn't have." Her hand moved to the back of her neck, rubbing it absentmindedly as she tried to find the right words. "We were so damn naïve. I swear, we never meant for people to get hurt..."
"Donna," Her flatmate's voice was steady but her body was tense. Her fingers were clutching to the material of the sofa so tightly that her knuckles were turning white. "What did you do?"
Donna's eyes slowly lifted to meet the other woman's stare, her breath leaving her in a rush as she forced the words out that she hadn't uttered in years. "That night, we killed a man- we killed Jake's uncle."
Give them all a go and see which works best for you! Have another suggestion? Make sure to let me know!