First, I think you should have an idea of what you want to study before you decide you really want the position. Select a few topics that might interest you, then read some papers on those topics, find out which faculty member has done similar research, read their papers, contact them, ask for their advice etc.
Second, ask yourself why do you want to enrol for a PhD course? To postpone the job-hunting and continue your student life? To increase your chances in getting a better job afterwards? Other?
You will probably be asked this at the interview. Good luck!