Refuelling is a matter of timing really, depending on how
close it is to the Stated time of Departure.
The closer the A/C is to it's STD, the quicker the refuel
gets started. Hopefully.
Refuelling from hydrants is quite straight-forward, whereas
DEfuelling isn't, and will need a bowser being "Available" to
accept the fuel coming off the aircraft. Which is not always
a straight transfer onto another of the Company aircraft.
Refuelling, defuelling with Pax boarding/de-boarding is
allowed but subject to many ifs and buts!