Since the philosophers of Greece, in Western civilization sages and learned folk alike have discussed what kind of attributes epitomize humanity. It wasn't until the Early Modern period that the notion of a human was challenged with the introduction of the automaton, the most famous being The Turk. It was the first in a long series of inventions that made humans of the age question the nature of humanity. Inspiration, intrinsic genius, these were all things that the well educated attributed to what composed a human being. A.I., or artificial intelligence, and the defining logic intrinsic to the computer language are concepts that we often take for granted. As we grow more advanced as a species, we continually slim down the qualifications for what can be qualified as a human. In the days of the Automaton, it was a fantastic idea for the Early Moderns to contemplate a world where things non-human perpetrated the human.
This idea is the foundation of science fiction and philosophically heavy writings. A little book by Henry Mackenzie laid out a protagonist that cried so much in his emotional exploits that an "Index to Tears" was added in the 1886 edition, much to the ridicule of the book's sentimentality. I reference this text because Mackenzie attempted to relate a human character riddled with feelings and emotions, but ended up with something resembling a robot more so than a human. What are emotions when they are reduced to merely being a cathartic reaction to sensory stimuli? Like a basic computer, Mackenzie's leading man Harley is nothing more than a flesh computer processing input data and spitting output emotions. It wasn't until the Romantic period of Wordsworth, Coleridge, Lord Byron, et al, that sentimental writings were taken seriously.
In a work of literature building off principals rooted in Western Civilization, there are many roads to take that can exhibit a character's capacity to feel and express. Asking, "How is my character human?" is a good start, because it forces you, the author, to contemplate basic needs and desires that found your protagonist. It also draws a distinction between the protagonist and the antagonist. The protagonist is the definition of human expression in the tale and the antagonist his antithesis, the "other". In H.G. Wells's The Island of Doctor Moreau, the protagonist Edward Prendick observes the blend between man and animal, thereby begging him to make the distinction of what defining characteristics determine Man and Animal. Is your character inhuman? If yes, then what characteristics make him a subversion of humanity?
Grappling with this dichotomy is what drives Early Modern Literature. As philosophy steadily deviated from divine epistemologies, from God ordained to derived observation, the answer to what makes humans intrinsically human has changed. How your book wrestles with this distinction can help add depth to the work.
For Example, I've always liked expressing my characters against inanimate objects. The implied stillness of the material, but non-human forms, amplifies the humanity of the protagonist:
A door opens, and I see light. It embraces me. The dust floats above like cinders settling around a pyre in the early twilight. A chair by the dining rooms lays still, a shadow hiding its gnarled grain. A table, a single place setting, a wick and candle stick, come together as a still life in my kitchen. As I move between them my hand fades in between the luminescent shafts of the lamp burning in the corner. I breathe life into a world without. Cabinets hiding fragments, memories of warm fondness lay dormant. They do not welcome me as I sit. And I realize that I am alone. So very alone.
See what I mean? What is human? What does it mean to be human? Is it in context with society, or is it in the intrinsic nature of being animate? Think about these distinctions and inform me how it changes your perceptions.