Final answer:
Bill took on the role of a mentor to the rookies on his team, offering guidance and sharing his experiences for their learning and safety.
Step-by-step explanation:
The role that Bill took on to the rookies on his team was that of a mentor. As an avuncular figure, Bill provided guidance and shared his experience with newer team members to help them learn and stay safe. His approach was one where he used his own mistakes as learning opportunities for the rookies, which is a key characteristic of a mentor rather than a supervisor, manager, or trainer who might have more formal and official roles of oversight or education.