If it's a good old two-part interview, there'll be a motivational section, and a competency section.
For the motivational part, it's about showing you're motivated enough to give it your all, and complete your training. When I had mine, they asked me why I wanted to be a pilot, how a wing generates lift, how a jet engine works, if I had any interesting aviation-related news stories to talk about, and if I had any flying experience. Of course if you know your stuff, they know you're pretty motivated, but they could ask you anything.
The competency section is hard to give advice about, and being familiar with competency interviews, I knew what to expect, and the stuff they asked was plastered all over the Internet. Searching for "competency interview questions" on Google will sort you out, so you can plan accordingly. They want to see key competencies, which you need to complete the course and succeed in the world of aviation, like leadership, problem solving, same old stuff...
And be yourself, you just make a fool of yourself if you lie!
