A psychological thriller is defined as a fictional story that focuses on the unstable psychology and emotional states of characters. It often involves elements of mystery and drama and explores common themes of reality, perception, mind, existence, purpose, and identity. Key conventions include deceptive mind games, flashbacks, tense music, shadows, and confined spaces. Cinematography techniques like low-key lighting, quick camera movements, and montage editing are used to build tension and highlight a character's emotions.