Some details are definitely more important than others, though. For instance, you should know:
1. What your character looks like,
2. What his/her/its personality is,
3. What your character likes or can't stand,
4. How you character speaks (this will help with dialogue).
Beyond that, anecdotes from your character's backstory are icing on the cake. They might answer how your character's personality formed, or explain a certain dislike, but it's important not to get bogged down in the character's backstory.
Which would you rather read? The first is an example of my main character from "Paradise Found", with tons of backstory. The second is the same character, but with the bare bones information included:
"Paradise Found" Backstory Example A:
Shane Carrigan just knew he was going to regret this. He'd been on the force for a couple of decades now, and should have known better than to let some civilian - especially a smart-aleck like this Michael guy - tag along.
Hadn't Shane learned anything from that ill-fated arrest five years ago? When a bust went awry in a densely-populated area, Shane had gotten the criminal, but not before three people died.
Ever since then, Shane had promised himself to work strictly within the rules. This Michael, with his uncanny knowledge of the murder investigation, was jeopardizing Shane's resolve.
Shane sighed and started toward the precinct's door. Michael stood there in the bull-pen flat-footed and uncertain until Shane called back, "If you're coming, keep up."
"Paradise Found" Backstory Example B:
Shane Carrigan just knew he was going to regret this. With all his experience, all of it instructive, some of it painful, he knew better than to let civilians play detective.
It didn't help that this Michael guy had a smart mouth and an arrogance that didn't match his age or apparent inexperience. But, the man had an unusually intimate knowledge of the murder investigation, and threats of prison hadn't shaken any of it loose.
Shane sighed and started to leave the precinct. Standing still as a statue, like a well-trained soldier awaiting orders, Michael didn't move until Carrigan called over his shoulder, "If you're coming, keep up."
Option B conveys the same plot information, but doesn't distract the reader with a piece of backstory that I don't plan to reference later in the novel. It's fine for the writer to know every detail of a character's life, but the audience is looking for an entertaining and rewarding read, which means skipping the less pertinent minutiae.
Take the following writing prompt and try writing it two different ways.
- The first way, include as much as you can about the character's history. Try answering all of these questions, and you'll see what I mean about providing too much backstory.
- Why is Jake working as a guard?
- Where did he work before?
- Was he fired, and if so, why?
- Do the other guards know his work history?
- The second way, focus more on the current plot, including just what you need to inform the reader about the character's motivations. Try answering these questions, and see if it helps you with furthering the plot.
- Why are the other guards mad at Jake?
- Does Jake have any experience that will help him overcome the situation?
Good luck, and keep writing!