Why it works
A role primes the model's tone, vocabulary, and standards. 'You are a senior security engineer' pulls in caution, threat-modelling, and precision that 'help me with code' never will. Production agents (Cursor, Devin, Claude) all open by establishing an explicit persona.
How to do it
Open with: 'You are a <seniority> <domain> expert.' Be specific about seniority and domain — 'senior B2B copywriter' beats 'writer'. Add a one-line standard if it matters: '...who writes concise, jargon-free copy.'