Prose craft
Free indirect discourse
Third-person narration in the character's own voice, without 'she thought' tags; narrator and character merge.
Free indirect discourse (or free indirect style) is third-person narration that renders a character's inner voice and diction without a reporting tag like "she thought that". The narrator's and the character's voices merge: "The party was unbearable. Why had she even come?" is free indirect discourse; "She thought the party was unbearable" is reported thought.
Jane Austen is the early practitioner the form is associated with, and the modernists (Woolf, Joyce) extended it. It is the workhorse of close third-person fiction and the natural companion of show, don't tell: it tells, but in a voice that shows.