Anyuway, first up would be to mention any previous experience in this sort of job.
After that the most obvious qualities would be efficiency and adaptability. Worst case scenarios would involve having to buy Christmas presents for your boss's wife/husband, having to work late, making the tea etc. With luck you'd get a better boss than that, but you never know.
You need to look though the person spec beforehand and plan how to show you meet each of the essential aspects in terms of skills, competences, qualifications and experience
I'm currently a PA and pretty much do all the things in Jno's link. One I would add that's critically important is to be totally trustworthy and keep confidences, especially as you usually have access to the boss' e-mail.