From EU-OPS:
Re/defuelling with passengers embarking, on board or disembarking
An operator must establish operational procedures for re/defuelling with passengers embarking, on board or disembarking to ensure the following precautions are taken:
1. one qualified person must remain at a specified location during fuelling operations with passengers on board. This qualified person must be capable of handling emergency procedures concerning fire protection and firefighting, handling communications and initiating and directing an evacuation;
2. a two-way communication shall be established and shall remain available by the aeroplane’s inter-communication system or other suitable means between the ground crew supervising the refuelling and the qualified personnel on board the aeroplane;
3. crew, staff and passengers must be warned that re/defuelling will take place;
4. “Fasten Seat Belts” signs must be off;
5. “NO SMOKING” signs must be on, together with interior lighting to enable emergency exits to be identified;
6. passengers must be instructed to unfasten their seat belts and refrain from smoking;
7. the minimum required number of cabin crew specified by OPS 1.990 must be on board and be prepared for an immediate emergency evacuation;
8. if the presence of fuel vapour is detected inside the aeroplane, or any other hazard arises during re/defuelling, fuelling must be stopped immediately;
9. the ground area beneath the exits intended for emergency evacuation and slide deployment areas must be kept clear; and
provision is made for a safe and rapid evacuation.
Ryanair's Refueling Supervisor procedure comply with these points, but it's always up for interpretation. Commonly you establish communication and then go for your walk around, wait outside till fueling is complete, sign the receipt and go back up. Certain airports, Italian ones typically, apply the regulations more rigorously and requires the supervisor to on the headset throughout the refueling process.