Interiority is a characteristic of being private, inward, or introspective. A writer can convey her characters' interiority by describing their innermost thoughts.
The interiority of a person is their quality of being focused on their own inner nature or musings. Certain things are so personal that they carry their own sense of interiority — think of the emotion of love or the faith of a religious believer. You can also use this word to mean "inside" or "protected from the outside," like when you admire the interiority of an enclosed courtyard that's sheltered from the busy city around it.