People live in context, and are a result of the choices they've made, the way they grew up and what they've done with their lives. In other words, we are "me-centric". We have our own point of view and the world exists--for us--through our point of view.
A only child college kid in an upper middle class household might view the world as her oyster. Daddy bought her a Miata for high school graduation, she's going to University, has good clothes, and great health--her teeth are white and straight, she's got a nice haircut and for spring break, she's touring Europe. (taking it as a given she's well-adjusted, lol)
"I love life! Life has always been good to me. I have no worries and my swim team is going to Tokyo during the summer."
but...if you take that same nineteen year old and put her in different context, the world changes because the focus shifts.
This kid grew up in a single parent working class household, she shares her room with two sisters. They have bunk beds and she has a twin. Her grades were bad because she worked at McDonald's to help her mom out, and she's tired all the time. Sometimes she thinks of going back to school, but somehow she fell through the cracks.
"I wish I'd never been born. My mom works all the time, and I'm tired of taking care of my sisters. People laugh at me because my teeth are crooked and I work at McDonald's. The first thing I'm going to do when I win the lottery is get braces."
The same sun rises in the same sky when each kid gets out of bed, but for one kid, each day has unlimited potential. She gets up, thinks about eating and decides against it because she's trying to stay a size 3. The other kid pulls her uniform from the shower rod, makes a face because it smells like mildew and decides against eating because there isn't enough for three.
They were both born with the same basic equipment in the same way each character starts out as a blank page, but the girl going to Tuscany isn't the girl getting written up for a dirty uniform. The choices you make about "who" your character is, influence "how" they act.
Did you think the rich kid buys lottery tickets? A million dollars doesn't buy much. But the kid with one discretionary dollar knows a million dollars isn't a million dollars--it's toys for her sisters, a car for her mom, an end to baby-sitting and a kind of freedom the other kid doesn't know she has.
Knowing your character's social status in relation to the world they inhabit is a necessity.
IMHO, it's the most important of the character building blocks because from the social background you also get physical and emotional building blocks. Each block leans against the other.
Genetics play a large part in physical traits, but you can create traits by tweaking context, although it helps to know "what" your character looks like.
Let's take a scrawny rat-faced villain and build backwards. Scrawny? How to make him scrawny? Let's malnourish him in early childhood. Which makes him either an abused or "poor" child. I'm going with poor, so--check. Background. Poor. Rat-faced? Lots of nationalities have the characteristics, but I think Dickens when I think "rat-face" so I'm going with some kind of Irish, Scottish, Welsh or English. Too bad he's American. So...let's extend that further. He's a poor man, of Scottish, Irish, Welsh or English descendant in the United States, which means his family probably came with the big migration waves in the eighteenth century. So now he's poor, Scotch Irish and his family landed on the Eastern Seaboard, see where I'm going with this? He's got a common Scotch Irish name like William which we can easily change to Willie, and he probably has a lot of traits associated with growing up poor. Which brings us to emotional building blocks.
The emotional truth of your character determines how he'll act under stress. Let's go back to Willie.
He grew up poor, he's probably got security issues. Which brings us back to physical--maybe he slouches, and gives people furtive sidelong looks. Maybe he had to have everything in his name, out on display and wears designer clothes. Is he trying to prove himself? What does he want? What type of childhood experiences did he have to act the way he does? Was the freezer always empty? Then it stands to reason he'll do anything to keep that freezer stocked. Did he watch people treat his mom badly because of her socio-economic status? Then maybe he has a sub-conscious hatred for poor women because they remind him of his mother and his former powerlessness.
You build layers by figuring out "why and how". A character is more than the role they play in your story.
You can't stay away from your characters, you have to be willing to dissect yourself before you can dissect others. A major problem in emotional layering is not creating the right character...or--finding the right story for that character. Characters and stories are not interchangeable. Why is "this" character in "this" story? What do they bring to it someone else can't?
tell me why your protagonist belongs in your story. Go beyond the cliched "well, it's his story", or "because he falls in love with the heroine", or "he's the only one who can stop the killer". Why "this" character in "this" story? What are you trying to show?