I went for AC last year and was up against people of pretty good calibre! Most of them either had experience working in government and/or overseas internships. I think that's what they are looking out for. I feel like they also want people who have a passion for public service - it's easy to tell if someone is faking it. Presentation skills and general knowledge about industries, economy and global current affairs would give you an advantage. I got rejected in the end but it was a good experience!