Just be yourself, remember the competencies they're looking for and work as a team. You're not being assessed on whether you complete the tasks, you're being assessed on how you handle the tasks set before you. Try and enjoy it and get to know the other people in your group as there's a chance you'll end up at Tulliallan with them.
What I will say is that as much as you'll be looking for someone to explain exactly what happens during the AC, you'll find that once you've passed you'll be hesitant to explain everything to those still to face it. And there's a good reason for it.
Best of luck :-)