Just to follow up on what has been said by SW1...
From my personal experience, there are two types of questions that they throw at you. The first type are the "must-know" questions, which is self-explanatory. You are expected to display a good level of understanding in certain topics, and the breadth of your knowledge pool should be directly proportional to your experience levels. Such topics could be ground school subjects relevant to your licenses/ratings, and aircraft-specific knowledge.
The second type of questions are to try to throw you off, see how well you work outside your comfort zone. I hold a PPL, with all my time being on single-engine aircraft and VFR only. I was asked to explain FLEX thrust settings on the Airbus aircraft, use of trim tanks for CG-related purposes, all the V-speeds etc. They also asked me behavioural-type questions, such as what I would do if confronted with a certain situation in an airline operation.
Just study as much as you possibly can, get in there and give it your best shot. Good luck!
