Character design involves an entire plethora of details and construction, including background development. Where many writers run aground is regarding how much background to devise and include.
Hemingway’s famous Iceberg Quote advises using only ten percent of what you know about a character in your story. The other ninety percent should be reserved for your knowledge base alone.
However, it’s easy to crash into that iceberg rule after you’ve worked so hard to round out a character.
How, you may ask, do I know what to use and what to leave out?
The only answer I can provide is, “It depends.”
What kind of story are you writing?
How long is it? A short story or a novel?
How complex a character are you designing?
What story role will this individual play? Protagonist? Antagonist? Sidekick, etc.?
Is background motivation necessary to advance the plot?
John Grisham’s breakout novel, THE FIRM, provides no background on the protagonist Mitch other than a couple of sentences about Mitch’s brother serving prison time. Background isn’t necessary here because the book doesn’t need Mitch’s past. He isn’t motivated by what’s happened to him before the story opens. Instead, the novel focuses on the mystery of what’s really happening in this law firm where Mitch now works.
In contrast, the Dick Francis mystery, ODDS AGAINST, features a protagonist whose past is doled out in bits from start to finish. Sid’s background is important because it feeds his motivation through the ongoing plotline. Francis is careful, however, to avoid info-dumps and never stalls the story advancement to indulge in them.
The two novels demonstrate how wide background material can range.