Truer words have never been spoken. Perhaps it was my mentor who told me that when I started writing.
Backstory should facilitate your character, not define your character. Backstory is sugar in the cake. What your character does during the story is the cake.
Forget the brooding, tortured soul. It's an overused and oh-so-familiar archetype so many people are annoyed with for that very reason. The only thing defining the character is their past. They don't do anything of value in the story. They sit there and brood, re-hashing the past to the reader with their backstory. Unfortunately, this is NOT the character to have.
A character can definitely have a tortured past, but they also have to do something substantial during the story. Many writers seem to think giving a character "a past full of dark secrets" will make them interesting. Many readers find this mega-dark secrets thing… well, boring. Think about it. Doesn't it seem logical it would be more interesting for the character to generate those dark secrets during the story process?
Let's analyze why this dark secret/tortured past scenario is so boring. What is the most common way for people to get across this past to the reader? They use: a) Flashbacks or b) The character bluntly tells us what happened to them. Listen. Boring!
Flashbacks are not effective at holding our attention because it slows the forward momentum of the story by going back to rehash the past. As a reader, we don't want to stop that momentum. We want to keep going forward. Flashbacks don't progress the story other than providing trivial or, sometimes, vital information. There's no action associated with a flashback since it moves the reader to another period of time in the past, rather than forward.
The character bluntly tells us what happened to them.
Talk about telling rather than showing! By doing that, it automatically is dull, flat and boring. Period.
You can definitely use flashbacks of the backstory, but attempt to keep it to a minimum. Every character has a backstory, so don't think this means you need to leave out a character's backstory. A cake without sugar would be tasteless. Also, a cake with too much sugar would be sickeningly sweet—you need to find that happy medium of how much backstory is needed. Remember, it will be different for each story and each character.
Backstories can be the vehicle to show your character’s motive. How? The most important thing to making an interesting character is for them to participate in the action of the story, to move forward with a motive. All characters have a history. If a character’s parents were killed, have the character take action to actively find the killer and enact revenge. Even better, have the character help someone else who experienced the same thing. If the character has made mistakes in the past, that’s fine. Let the character make amends to fix those mistakes.
Backstories are a double-edged sword. It is your job to hone it properly without slicing up the story.