What is it?
It's making a character feel like a person. Like you could relate to them or find them out in your world pretty easily.
Let's dispense with the bullshit of "it's so hard" and go give you some practical tools for it.
Go get some paper and your character.
Name the character AFTER you've built them.
Are they the female protagonist? The third POV character? The dudebro wizard in your urban fantasy?
Identify them objectively, because thinking that way helps you be critical about them.
Nope, that's the icing on the cake, not the cake itself.
No one's gonna give a shit about their haircut unless it's the center of the story. Save that for later.
Answer this - WHY IS THIS CHARACTER IN THE STORY?
Make a list of all the answers, in any order, WITHOUT JUDGING THEM. Just write them down.
And that's great reason to be in the story, but it's not gonna be the ONLY reason to be in a story.
Let's dig a little deeper.
"WHAT DOES THIS CHARACTER HOPE TO ACCOMPLISH FROM THE TIME THEY ENTER THE STORY TO THE TIME THEY LEAVE IT?
Again, some of this might be plot, and that's okay. But not all of it. Don't let it be all of it.
Because that's the role of the characters we don't give a shit about it. Like the waiter in chapter 7. They do this one job and then they melt away. There's not a lot to develop there.
Yes, maybe the plot or accomplishing the plot is A goal, but it's not the only thing they've got going on -- it's just that the story that you're writing is about this plot and this one goal of theirs.
But that's NOT going to help your characterization.
We want this character to be fully formed without crutching on you. There's enough you in them already.
This is an object, it's outside you.
Treat them like they're sitting across from you as you're writing.
HOW ARE THEY GOING TO ACCOMPLISH THE GOAL?
Note: This is not a plot question. If you start rehashing plot, start this question over.
The question here - what skills can the character apply to the situation to get the goal?
A list that maybe you'll eventually rewrite by order of skill quality (they do this best, they do this second-best, etc)
But list all the things they can do, INCLUDING the stuff that doesn't show up in this story.
WHAT DOES THIS CHARACTER FEAR? WHAT AREN'T THEY WILLING TO DO?
WHAT DOES THIS CHARACTER SWEAR UP AND DOWN THEY CAN'T DO?
They're afraid of rejection. They're afraid of geese. They don't want to die alone. They can't climb trees. They can't commit to a relationship without decent WiFI.
"Oh I too am also afraid of geese." and you build a bridge to them.
Help them find themselves in your work, just as you may find yourself in there too.
The world, whatever your world is in-story, is going to present obstacles to them en route to that goal.
If Mary really wants to take Janine to the prom, but Janine is all googly eyes for Scott, then Scott is Mary's obstacle.
What they will/won't do (let's call that a Philosophy)
Now go back and physically describe them. See how their description is an independent casing for their complex sausage.
(note to self 2: oh shit @_Thaddeus is checking out your shit, don't freak out, just be cool yo)
So if that's how you build a character, what do you do with them, aside from "GO WRITE NOW" ?
One more thing.
No, doesn't matter if you know ALL of them. Maybe you do, maybe you don't. That's fine.
Maybe it looks like:
Carl goes to work
Carl kills a man with a burrito
Sally plans revenge.
And oh sweet bell peppers don't get all precious and call this an "outline" because that's a loaded term.
Let's call this "organization" because what you're doing.
And make sure that in that scene, they're doing something towards their goal(s).
Then this scene at work is a chance for him to fantasize about that, and relay that goal to the audience.
If the scenes don't provide opportunity for characters to accomplish or move on their goals, what's the point and why is it on the page?
Maybe not as specifically as "I too want to be an assassin" but maybe in that "I spend time at my dayjob wishing I was doing something else."
You're aiming for "I feel you [CHARACTER NAME]"
Please note at no time did I insist that you dread this, or make a mountain from this not-mountain-that-really-just-wants-you-to-make-some-decisions-about-ways-other-people-relate-to-your-ideas
Take your time.
Answer the questions and don't settle for the quick answers. Be willing to dig a little and not settle.
Push past the kneejerk reactions.