Hiya! I run my own PR company so haven't been interviewed for a couple of years but I would say now to massively brush up on all things social media. The lines between social and PR are so blurred now that they need to integrate seamlessly.
Find a few recent campaigns that have done this really well and not so well (generating bad press in the latter case) so that if they ask you for examples you can confidently talk them through. Always use examples they definitely haven't worked on if it's an agency role! Mention things you've worked on before that worked well but be succinct so you aren't just reading your CV!
I remember being asked a few times what my dream client / campaign would be / what campaign I wish I could have worked on / most successful or enjoyable campaign I have worked on.
Also it sounds really basic but go through the company's website really thoroughly so you can mention their clients / ethos / news etc. Never be afraid of sounding too keen in a PR role.l but again be succinct and don't waffle.
I've often asked candidates how their colleagues would describe them, then how their clients would describe them. Have found this a useful way of getting past stock interview answers but of course don't know if other interviewers ask this!
I'd say that when it comes to PR interviews, the key traits you want to show off are tenacity, creativity, strategic thinking and communication skills.
Good luck!