What LoLevel said is technically correct, however the Flight Plan creation that is mentioned is generally done at the flight planning stage. Pop Ups here in Sydney are handled by a planner position, with one planner serving between 5 & 6 "Executive" positions. The idea of creating flight plans while say working an approach position would be irresponsible not only from a system but a personal level. I assume LoLevel is a centre controller and could enlighten us to the enroute scenario which is certainly more of a pain with regard to "filed in flight".
The planner position would be closer to the assistant position in that it has some coordination responsibility. It is always manned by a licensed controller who is co rated on the associated executive position.