
Whereas character reveals the deep-seated nature of who people are, in terms of values, actions, and beliefs, characterization is expressed in the way people live, the cars they drive, the pictures they hang on the wall, their likes and dislikes, what they eat, and other forms of individual character expression.
Syd Field, Screenplay: The Foundations of Screenwriting (2005, p. 55)
Characterization is the sum of all observable qualities of a human being, everything knowable through careful scrutiny.
Robert McKee, Story: Substance, Structure, Style, and the Principles of Screenwriting (2010, p. 100)
We may divide characters into flat and round. Flat characters were called “humorous” in the seventeenth century, and are sometimes called types, and sometimes caricatures. In their purest form, they are constructed round a single idea or quality: when there is more than one factor in them, we get the beginning of the curve towards the round.
E. M. Forster, Aspects of the Novel (1927, p. 67)
All three-dimensional characters, when we first meet them, are flawed. In psychological terms they are victims of neurotic trauma; there is a mismatch between their wants and needs…
John Yorke, Into the Woods: How Stories Work and Why We Tell Them (2013, p. 145)
Flaws are a starting point of imperfection or incompleteness from which a character can grow.
Christopher Vogler, The Writer’s Journey: Mythic Structure for Writers (2020, p. 36)
A three-dimensional character is the foundation of all good plays. You’ll see perpetual transition in their work. And above all, you’ll find direction: a clear-cut premise.
Lajos Egri, The Art of Dramatic Writing: Its Basis in the Creative Interpretation of Human Motives (1942, p. 262)
A novel that is at all complex often requires flat people as round, and the outcome of their collisions parallels life more accurately.
E. M. Forster, Aspects of the Novel (1927, p. 71)
Like female characters in the Hero’s Journey, male characters in the Heroine’s Journey…are often undeveloped archetypes of masculinity.
Gail Carriger, The Heroine’s Journey (2020, p. 95)
Caricatures are uninteresting in literature, perhaps, but have their purposes in politics. Flat characters make for better political narratives, precisely because they display simple emotions and their audiences feel simple emotions about them.
James M. Jasper, Michael P. Young, and Elke Zuern, Public Characters: The Politics of Reputation and Blame (2020, p. 27)

Who is your story about? Separate the components of his/her life into two basic categories: interior and exterior. The interior life of your character takes place from birth up until the time your story begins. It is a process that forms character. The exterior life of your character takes place from the moment your film begins to the conclusion of the story. It is a process that reveals character.
Syd Field, Screenplay: The Foundations of Screenwriting (2005, p. 48)
When designing a character, its often useful to think of them in terms of their theory of control. How have they learned to control the world? When unexpected change strikes, what’s their automatic go-to tactic for wrestling with the chaos? What’s their default, flawed response?
Will Storr, The Science of Storytelling (2020, p. 68)
If a character doesn’t want something, they’re passive. And if they’re passive, they’re effectively dead. Without a desire to animate the protagonist, the writer has no hope of bringing the character alive, no hope of telling a story and the work will almost always be boring.
John Yorke, Into the Woods: How Stories Work and Why We Tell Them (2013, p. 9)
When we plant an acorn, we would be justified in expecting an oak tree and shocked (at the very least) if it turned out to be an apple tree. Every character a dramatist presents must have within it the seeds of its future development.
Lajos Egri, The Art of Dramatic Writing: Its Basis in the Creative Interpretation of Human Motives (1942, pp. 67–68)
Subtext then emerges from the interaction between a character’s façade and their actual intention or goal…As want collides with need, the greater truth—the gap between what characters say and do—is revealed.
John Yorke, Into the Woods: How Stories Work and Why We Tell Them (2013, p. 164)
The test of a round character is whether it is capable of surprising in a convincing way. If it never surprises, it is flat. If it does not convince, it is flat pretending to be round.
E. M. Forster, Aspects of the Novel (1927, p. 78)
It is possible that at a moment of frustration and despair a man will do the unexpected in real life—but never in the theater.
Lajos Egri, The Art of Dramatic Writing: Its Basis in the Creative Interpretation of Human Motives (1942, p. 135)