What is role or persona prompting, and when does it actually improve model output?

Role prompting assigns the model an identity or expertise, e.g. "You are a senior tax accountant reviewing a filing." It works by steering tone, vocabulary, level of detail, and which conventions the model applies, since it conditions on text patterns associated with that role. It reliably helps for style, audience, and format control (a doctor vs. a child explanation). Evidence is mixed on whether personas raise factual accuracy on hard reasoning tasks; benefits are often small or inconsistent. Best practice: put the role in the system prompt, then add concrete constraints (audience, format, what to avoid) rather than relying on the persona alone. Avoid overloading with multiple conflicting roles.