I also interviewed with them in the past and received an offer. The most important thing, in addition to the usual ones (Why the Firm, Enthuisiasm, Why consulting...) is to give them a sense that you know what the Public sector environment in Middle East (especially Saudi Arabia, as they are working mainly there) is about.
So they may ask you some cases that involves the Vision 2030, the key priorities for Saudi, the Approach to Turism, which are the key sectors that they should consider to growth the economy, How much is important for them to differentiate by oil...
Give them a sense that you have made your research on the key macro economic problems and trends in Saudi/ Middle East and youll be in a good trajectory.
Hope this can help,