"You are an expert" — does it actually help?
Persona prompts ("You are a senior software architect with 20 years of experience…") do a real thing: they shift the distribution the model samples from. They also do an unreal thing: they do not give the model knowledge it didn't have before. "Pretend to be a doctor" does not produce safe medical advice; it produces text styled like a doctor's, which is more dangerous, not less.
What persona buys you
- Tone — formal vs casual, terse vs warm.
- Vocabulary — domain terms appear naturally instead of being redundantly translated.
- Format defaults — "as a librarian" yields catalogs; "as a journalist" yields lede + nut graf.
- Refusal calibration — a "financial advisor" persona refuses different things than "a friend."
What persona doesn't buy you
- Accuracy. The model is not your imagined expert; it is itself.
- Authority. "Expert" doesn't grant new training data.
- Safety. Personas can be hijacked by user input.
Productive personas
Use persona to set tone and format defaults, then use the rest of the prompt to specify behavior, evidence, and constraints. The persona is the costume; the contract is the script.