The Martian exhibits a variety of narrative styles, from the more traditional third-person point of view to a unique use of the epistolary style. The epistolary novel is written as a series of letters or, in this case, mission log entries. Although he intersperses the epistolary passages with other narrative styles, Weir uses the epistolary to great advantage.

Watney is alone on Mars and has no one to talk to, yet solves complicated problems and implements untested methods (untested, at least, on Mars). He is experimenting. He is also lonely and facing the difficult prospect of preserving his physical and mental health while he works to get off Mars. The mission log allows Watney to express himself and track the efficacy of his endeavors without resorting to long soliloquies or interior passages where he is thinking. As Watney notes, these log entries may never be found or read, but they allow him to externalize his thoughts and feelings. This allows him to work through problems and feelings on the page, but it also offers the reader direct access to his interior world without having to spend the entire book inside his head.