What are the common interview questions and what should I be watching out for public service interview at mid-career?

There will most probably be stock questions. (Things like tell us a bit about yourself, what are your interests, what are your strengths, etc.) These are set pieces for most interviews and help the interviewers know a bit about you. You should think about your answers to those and at least run them over in your head once or twice so that they come out coherently.

In terms of packaging yourself, think from the interviewers' perspective: why would someone with decades of experience in the private sector be wanting to join the public service at this point? What passions/ interests are driving you?

Consider also how your passions tie in with the good of the public service. Your interviewers won't just want to hear you talk about yourself. They want to know why you believe their cause is important. People generally want a sense of importance, so communicate understanding and respect for their work and they'll know you mean business.