Flight Planner
Maybe try to find out what flight planning system they are using and see if you can learn something about it.
What aircraft are they using? Do some research on the type, engines, fuel burns, typical endurance, capabilities.
There may be some performance questions so know about the various weights (BEW/MTOW/ZFW/MLW etc). Know how they are derived and how they have an affect on the operation. The FAA weight and balance handbook is free to download online and is a good refresher on these topics.
Emphasise cost control particularly with regard to fuel savings. Shortest routes, optimised flight levels, closest suitable alternates etc
Understand the differences between LRC/HSC etc and if you can display that you understand how cost index flight planning is carried out then you're hitting the high notes. There are some good sources of info out there just search for 'Getting to grips with cost index' by Airbus.
Know your weather theory, particularly if you'll be doing some longer range stuff where taking advantage of jetstreams can be a great benefit but also avoid them when going in the other direction. Being overly conservative can mean routing around something that you could have flown closer to. Being too gungho can mean an unneccessary diversion.
Know how the MEL/CDL can have an affect on the route, level, payload.
Know about the AIRAC cycle, every flight planner needs to know about it especially in EU where there are lots of changes every month.
Know the hemispheric rule, brush up on RVSM requirements and MNPS.
You might get asked about overflight permits depending on the type of operation you'll be working in.
Those are the things I would ask.
Good luck