Characterization is such a big part of building a strong story. Your characters should feel real to you. You need to know them inside and out—their thoughts, feelings, motivations, and relationships to one another. If your story is in first person, knowing your protagonist is paramount.

But how do you achieve this level of intimacy with a character?

Much like world-building, you can ask yourself a million questions about the character. Some of these questions will directly inform your writing, illustrating motivations, personal history, personality, and relationships. Others will seem more trivial, but do not underestimate the power of getting to know your character through trivial details. Knowing these things will provide you with a more rounded view of who they are.

Personal History
Where were they born? Did they move as a child? Do they have siblings? Do they have children? Pets? What socioeconomic class are they in? What is their gender/race/sexual orientation/romantic orientation/religion? What obstacles have they overcome, and how did it change them as a person? What do they struggle with?

These questions give us a strong sense of who the character is, mostly in the context of society. When you ask yourself these questions, you also have to think about what it means in the context of the world. Is this common? In what ways are the people around them similar or different? Is this trait revered? Is it shameful, or even taboo? How open is the character about it?

Self
How do they feel about their body? What do they do for work? What are their aspirations? What are their passions? What sort of political bias do they have? What is their Achilles heel? What’s quirky about them? What do they look like? What do they value? What’s their athletic ability? What are they good at? What sort of responsibilities do they have around the house?

Knowing the character’s self—how they see themselves, their biases, their dreams, and the mundane factoids about who they are—are essential in creating a character. Especially in a first-person narrative, these color the way your protagonist sees the world, and thus the way react to the obstacles in their way and the way they tell their story.

Relationships and External Perception
What is their relationship like with the immediate members of their family? What kind of people do they gravitate towards? Who are their friends? How do they feel about them? What do they think is their greatest fault? How would their parent/sibling/child/love describe them? How would their best friend describe them? How would a casual acquaintance describe them? How would their arch-nemesis describe them?

While knowing how your character views his or herself essential, it is just as important to understand how others see them, and how they see the other people in their life. This gives a layer of objectivity, and informs the way they interact. Who’s opinion it is matters, too. For example, if your character values perception by the outside world, the opinion of a casual acquaintance may mean far more than a family member. If your character is a recluse, perhaps no one’s opinions mean anything, save for the one person they care about.

Categorization
What’s their Myers-Briggs personality type? What Hogwarts house would they be in? Are they type A or type B? What’s their Enneagram number? What’s their horoscope? Which of these does the character identify with and to what degree?

These convenient forms of categorization provide a personality profile. Almost more important than identifying these characteristics is determining to what degree the character believes in that form of categorization or identifies with it. For example, if someone were to read their horoscope every day, live their life by them, and avidly avoid all Taurus—that would tell you way more than just knowing they’re an Aries.

Preferences
What is their favorite book? Favorite movie? TV show? Who is their favorite band/musician/actor/writer/food/color? What’s their favorite food? What do they drink? What tasks do they despise completing?

These detailed preferences may not be the most salient aspects you need to know about your character in order to write the story, but do not undervalue them. These little details will flesh out the world in which they live. A favorite book, movie, TV show, or writer may influence the way they see the world. For example, if they like crime shows, they may perceive the world as being more dangerous. But if they like reading Emily Dickinson, they are likely very introspective. These details may also color their actions, making them rich with life.

More resources for fleshing out your character:
Lit Reactor
CW Guild
Winghill Writing School