The formal duties
Control access to the site, patrol on schedule, monitor CCTV where deployed, respond to alarms and incidents, complete daily occurrence books and formal incident reports, liaise with police and emergency services.
The unwritten standards
Uniform pressed and clean, boots polished, phone in pocket not in hand, greet every visitor within 3 seconds, refer to the client's staff by name where possible.
What separates strong guards
Report writing. A clear, timestamped, third-person report with specific descriptions is the artefact clients quote back to your agency months later.