futurecadet05, when I went for my selection, they asked quite a few technical questions a bit more complex than 'how do planes fly?'....it was a while ago but I remember talking about things like thrust reduction on takeoff. They also asked some non-techy questions for example about CRM, so be prepared for them.
I think what they ask is dependent on how you answered the questionnaire they emailed you.
To be honest, you shouldn't panic about the interview. It's more about getting to know you and your previous experiences (that relate to the qualities they are looking for in potential pilots) and to make sure that you can stay calm under pressure.
The number one rule I always stick by is: Don't freeze!....always always always say something, it seems obvious but I'm surprised by how many people who say nothing or just say 'I don't know'....I remember in my interview asking for a minute of thinking time before I answered a particularly tricky question, she didn't mind at all!
Just stay calm and be yourself and you will be ok
I hope that helps