Expository nonfiction and biography, even if both are written on the same topic, aim to achieve different goals. Expository writing aims only to inform the reader about a subject, such as how something works or what something means, while a biography is a narrative written by the author about another person, which informs and entertains. For example, an encyclopedia entry about President Theodore Roosevelt would provide a summary of facts with no narrative theme. A biography of Roosevelt, however, would certainly include facts, but it would tell specific stories about the things he did, such as detailed accounts of his hunting expeditions, or tales of his personal relationships.
A biography can either be book-length or shorter, but it is always a narrative told by the author about another person's life. Furthermore, it can assume any number of structures, provided it's a narrative structure. For example, that Theodore Roosevelt biography could detail his life from his childhood to the end of his life, or it could be told elliptically: It could start when he's already president and then back-track to recount how he became the adult he grew up to be. Exposition, conversely, can take any form designated by the piece's destination. Such a piece could take the form of a single-sentence photograph caption explaining a wildlife photograph, a newspaper obituary or a journal report on the effects of genetically modified organisms in foods.
Biographies rely entirely upon storytelling, which means that one will contain scenes, often with multiple characters whose conversations are shown through dialogue. Biographies can also contain thoughts and emotions: The author can include what a character might have been thinking and how that character feels at a given time, in order to convey a deeper sense of a character. Expository nonfiction can communicate information using narrative techniques, such as a chronological recounting of events, but it doesn't deal in intangibles. For example, expository nonfiction could be a set of instructions to install a ceiling fan, but won't discuss how you might feel during the installation. For this reason, expository nonfiction has more in common with academic discourse: clear, purposeful and logically ordered sentences are the standard for assessment.
Writers of both forms of are expected to write honestly. However, while the expository nonfiction writer must always relate facts, the biographer has the opportunity to work with subjective truths. For example, biographers often rely on personal interviews as much as they do upon traditional scholarly research. An account recorded from an interview may be the truth according to the interviewee, but it might also be incorrect -- either because of deception, faulty memory or circumstance. The subjectivity of truth is something that is accepted in the biography industry, as biographies attempt to offer unique angles of perception regarding their subjects, to show a side of the person not readily available to the public. Expository pieces, however, must be thoroughly researched and fact-checked.