A fire watcher's primary responsibility is continuous surveillance during high-risk activities like hot work (welding, cutting) to detect, prevent, and respond to fires, acting as a first responder by sounding alarms, alerting emergency services, and potentially extinguishing small fires while ensuring personnel safety and evacuation, focusing solely on fire watch duties without performing other tasks. They must monitor for ignition sources, maintain fire safety equipment, enforce protocols, and remain vigilant for at least 30 minutes after work ends.