It's a bread-and-butter writing question: how do you write characters? Every writer has different methods, and sometimes you use different methods depending on the story you want to tell. Here are just some of mine.
Background
A good old Dungeons & Dragons method, what is your character's background? Were they raised by the streets, scrabbling for coin and always fearful of starvation? Were they born with a silver spoon in their mouth, servants at their beck and call? Were they a traveling musician? Born with a disfigurement that made others fear them cursed?
While the D&D backgrounds are, of course, fantasy-focused, it can really help flesh out even a contemporary character. If you look here, you can see the various D&D backgrounds. One of my favorite things for small-time characters is the rolling tables for traits, like this one:
You can use this site to roll 2d8 (two eight-sided dice) and pick those two traits. (Refresh if you get same trait twice.) So, I rolled 4 and 2, meaning this acolyte is boundlessly optimistic and always working to find common ground between opposing sides. I can work with that!
The protagonist finds themself in a dark place, and seeks shelter at a small temple. There, they meet the sole acolyte, a short, apple-faced man who gives them a gentle smile and a hot meal, and points them to the light behind the clouds, raising their spirits, even if they aren't perhaps religious themself.
You decide where this acolyte came from—from means or from meager, why he joined the service of whichever god he worships, and bam, you've got a solid side character to help your protagonist limp their way through Act 2.
(And of course, D&D also offers tables for ideals, bonds, and flaws, which are also fun to incorporate if you want!)
Goal
Now, for a protagonist, or any sort of dynamic character, a background and personality aren't enough. The easiest way to make a character compelling is to give them a goal. Do they want to be the best, like no one ever was? Do they want more than this provincial life? Sometimes, what a character wants is actually different than what they need, but that want makes them compelling regardless.
So, what do your characters want? What are they doing to achieve it? Will they succeed?
Maybe their goal changes as the story goes on.
Star Sign
It can be hard to invent flaws that feel realistic without marring your perception of a character, but flaws are essential to crafting a solid, well-rounded character. So, when I make a character, I look through astrology and assign them a sun sign (that's the one people generally know).
In my current WIP about Dionysus, for example, I gave him a sign and wrote out the relevant bits of that sign especially.
Dionysus
Aquarius
+ independent, loyal to those close to him, stimulation-seeker, inventive
- unpredictable, emotion-driven, detached (live and let live, don't get close, don't get hurt), extremist (no middle ground)
You can see I took both some positive and negative traits, and expanded upon them, brainstorming how I might make use of them in the book. Notice he's very loyal, but also detached. So, he's selective in who receives his loyalty. Why? Well, maybe he was hurt in the past and/or had his loyalties tested.
Think about how traits might connect. If someone is empathetic, perhaps they have trouble seeing people as bad? If someone is a good listener, maybe there's not a very good speaker.
In a POV character, like a protagonist, be aware of challenges this method may present. For example, in the above, Dionysus is "unpredictable", but most people do not literally make decisions at random. If someone is unpredictable, it just means the path their thoughts took is obscure to you, an outside viewer. In a POV character, the reader has to at least be able to follow the thread of the character's logic, even if they themself would not make the same choices.
And finally, as a writer who only does a bit of planning…
Write the First Draft (or Character Studies)
Living in the protagonist's head is my personal favorite way to develop them. For me, it means the first draft is not only me writing, but also me reading, learning the routes and paths a character's thoughts take, how they react to situations.
For example, I learned Innes from In the Shade of the Tree of Life, was a skittish person, as well as prone to catastrophizing while I was writing out the opening scenes. So as I wrote further, I leaned into that, which made scenes of calm even more special, and scenes of Innes' fears coming true that much heavier.
Now, if you're more of a planner/outliner, character studies might be the way to go. Write short pieces with your protagonists. Maybe a scene from their childhood, or doing something mundane, or even go to an extreme non-canon place. Write a short coffee shop AU of your New Adult horror protagonists. Write them going to the beach. Whatever would help you to get to know the character and get into their head.
Some elements of this character writing might find their way into your manuscript. Maybe none of them will. But you will have a stronger sense of who exactly it is you're writing about.
So those are my personal methods of sketching out a character! If you have others, feel free to tell me about them!
Today's song is I Can't Decide by Scissor Sisters.