Recently I watched an interview in which a famous fantasy fiction author discussed the difficulties he'd had in writing his latest book. His main character had amnesia, so the author was struggling to draw on things to develop him.
Think back to a number of good books you have read. Most, if not all of them, will reference the characters' pasts. A classic example I can think of is Harry Potter. In the first page, Rowling describes Petunia Dursley's long habit of spying on her neighbours. She also describes, further on, how the Dursleys have always been opposed to the idea of magic. When she introduces Harry as an eleven-year-old, she mentions his asking Aunt Petunia about his dead parents in the past. All of these things show us specific things about the cast of characters. Petunia is nosy, and has little better to do with her life than eavesdropping and gossiping. Vernon Dursley is afraid of what's different. Harry's deepest desire is to meet his parents. All this in a few chapters! History is a valuable tool for developing characters.
I've read books in which the characters were really boring. And in numerous cases, the author never mentioned their history or their motivations.
When dealing with your characters' histories, you don't have to write a novel of background. But the reader needs to feel like you know. You should be in possession of all those little facts that the reader doesn't have. What the reader receives should be crucial and add to the story. But it should just be the tip of the iceberg.
Once you've mapped out your characters' histories in your own mind, you should get into the habit of mentioning them. You don't have to say much. Just a little something to add to flavour, to show us something about this person's desires, motivations, and habits. Here's a couple of examples:
"Bertha couldn't stand flowers. She hadn't been able to abide them since her little boy's funeral.
'Thank you, but I'm allergic,' she said, thrusting them back at Paul, who abruptly looked crestfallen."
"Jonah stared with disgust at the tickets Aunt Mabel had given him. He'd only ever been to an opera once before, and it had been disconcerting. The soprano had looked like an overgrown turkey, and Jonah had been the only one throwing tomatoes. It had made him feel very alone."
These sorts of things will convince the reader you know your characters. The reader wants to know they're in good hands. But most importantly, the reader wants to feel these characters are real people. They have desires, motivations, and grievances. They have existed before the reader opened your book. And they will continue to exist after it is shut.
Characters are forever.