Makings of a Hero
As a DM, once your world is fleshed out and has an over-arcing theme that will drive the narrative, you need players (PCs) to fill that world. Who will those heroes be? Typically it'll be your friends, so you have the luxury to help them craft a character that fits within your realm. The PCs will develop their own ideas of what they want to play, a magic user that hurls fireballs or transfigures the world around him or perhaps a hand-to-hand brawler. The class and race of the character is easy, but if they are just in your world with no purpose, you might end up with the regrettable campaign scourge that is the traveling murder hobo.
This is why crafting the character with your players is key, you can give them context of the land, its history, its politics, and everything else that will paint the picture of what the PC was doing before the story began. It is equally as important if you are writing a story to flesh out your characters just as much. As the story is laid out and events unfold, the characters will then have purpose ti interact with those events and draw the narrative along.
In writing a story, as a reader we want characters that have lives before the "story" begins. We don't necessarily care that they were a blacksmith or a traveling bard, but we do care that they have wants and needs that make them interesting. Something we can relate to, empathize not sympathize. So we need to ask ourselves, as a PC, as a DM, as a writer, "What makes this character fascinating?"
The back story paints us a picture of what happened to the character beforehand, it shaped them into the character we see now so that we can see how much they change by the end of the story. It could be the driving force of their actions.
"My family was murdered by the passing army of the south as they advanced to the capital, and so I joined the Legion in order to avenge them."
"I was raised an orphan" - a trope in my group of friends/PCs, they probably read The Thief Lord as kids - "And so I was raised on the streets by a Thieves Guild, but I was setup to fall by my comrade as he took the seat as Thieves Guild Master."
These make the character interesting and can give us an idea of how they will act in certain situations. It makes them fascinating to read or watch as they make their way through your campaign. It's critically important that you help your PCs craft this background as it'll make them more invested in the story, which is also the case when it comes to a reader of your stories, for you authors out there. This process will set you up for success for throwing a curve-ball in to the story.
Does the PC meet the general that killed his family so many months ago? How does that PC react? Does he murder them and avenge his family, or does he have a change of heart after seeing so much destruction after countless battles in the Legion?
The entire point of a campaign or story is to see the character progress and see how they react with the story that is unfolding before them. Ultimately to come to the conclusion as we see them react, change, and develop as characters. To see this change the PCs need two thing, a moral need as well as psychological need.
Moral Need = alignment grid (Chaotic Neutral, Lawful Good, etc) - How do they react in difficult situations?
Psychological Need = Aspirations (Revenge, fight in the name of their Deity, etc)
The Psychological need will drive them along the story line while the moral alignment will paint their actions along the way. A lawful good character will stand up for justice every time, despite the odds stacked against him, perhaps because his deity commands him to (think Paladin). Knowing these two characteristics will make the character who they are at their core, while the backstory gives us an idea of where they came from and sets a benchmark for where they can go. If you've done your due diligence, either partnering with your PCs to find out what this is or crafting it yourself for your story's characters, then you as an Author/DM will know how to drive an interesting narrative.
At the end of the day, we play this game or read stories because we want to be in the shoes of these characters. We want to go on an adventure, we want to be the hero of the story.