Think of some of your own relationships—in the following example I’ll refer to a girlfriend, but it could apply to any close relationship.
Remember how you met and first became friends. You didn’t tell each other everything right from the get-go… You established that relationship based on certain qualities that you liked about each other. And as the relationship progressed perhaps you discovered a few other things you didn’t like quite as much. But you accepted them as part of who this person was—a direct consequence of her backstory.
Yet through all of this, her backstory remained in the background. You learned to appreciate this person despite not knowing everything about her past. Rather than define your relationship, backstory gave it flavor. That is the critical distinction.
For the sake of argument, let’s say your girlfriend was repeatedly abused as a child. What if she revealed this to you on your very first date? Without passing judgment on anybody for the way they respond in such situations, reactions here vary wildly from one individual to the next: they may range from a surge of compassion to mild indifference to, “I’m running the hell away from this person.” That’s just how it is.
To continue on this trajectory, let’s imagine you stuck around after she revealed her heavy backstory. She doesn’t know if it’s because you’re naturally compassionate or if, like Tyrion Lannister, you “have a tender spot in your heart for cripples and bastards and broken things.” You have your own backstory that shapes the way you respond to events in your life. But she doesn’t know that. The end result? She’ll be surprised that finally someone sympathized with her; certainly she’ll be intrigued, and she’ll want to learn your backstory at some point.
And so it goes for fictional characters.
Backstory falls in the “nice to know, but not crucial to developing this relationship” category. No matter how you slice it, by revealing backstory at the wrong time, you are gambling with your audience’s reaction, for the sake of something that shouldn’t be all that important to know in the first place.
(If you think it is crucial, you might have started your story too late… or, more likely, your relationship is missing that spark that would make it work without requiring an info dump.)
More importantly, dangling the carrot of backstory can be a powerful hook to keep your readers turning the page.
Now fast-forward three months, six months, a year. You really love this woman. Her quirks (and yours!) have helped shape this relationship. She finally decides to open up about her very painful past. Is it conceivable that your reaction includes mild indifference or, “I’m getting the hell out of this relationship”? I suppose it is, although that is highly unlikely.
Let’s take it one step further and imagine that she tells you all this in response to some hardship you’re going through as a result of your own backstory. More than likely, you have just formed an extremely powerful bond that will be nearly impossible to break.
Establish a powerful emotional connection between the character and their audience first (this includes your readers!). Make your character’s audience relate to that character.
Only then will your character’s backstory matter. Do not gamble with your audience’s reaction. Deliver the gut-punch when you know for certain it will make them cry. No sooner, and no later.