I applied for security guard a few years ago. You will be asked to watch a short video showing a few security issues, and will then be asked to write down all the issues you saw. Dont worry, this is easy, just things like a van parked right next to the airport fence, with a ladder truck thingy on the other side.
You will then fill in the standard forms, including Counter Terrorist Check forms, when filling this in i would recommend not ticking the "are you related to terrorists" or "have you ever plotted to overthrow the government" boxes.
After that just a standard interview, questions like do you prefer working in a team or individual, give examples of where you have shown leadership, examples of showing your own initiative etc etc
I didnt get the job,hope you do better than i did...