Yep - several. I assume you have applied for a specific project.
Show you know the subject and be able to explain why you are interested in this particular project highlighting any previous research/study you've done that's relevant. Be curious and enthusiastic. Check out your supervisor's areas of interest and know a bit about that. Think about any questions you have esp anything relevant to getting the project done or further training/development goals.
At one interview, with no notice, I was also shuffled round and interviewed for a couple of projects I hadn't applied for and that I did not want so had to be diplomatic - enthusiastic generally but very clear my interest was specific. Worth knowing what else they've got and why/why not you'd do them.
Use the interview to assess whether it's the right fit for you too. You are going to have to work with this supervisor closely and spend 3-4 years thinking of nothing else. What's the rest of the team like - you'll probably share a lab? And the department? Take a tour if offered. Think about whether the supervisor has the experience and resources to support the project? Some projects will be very tightly specified, maybe part of a bigger research drive, others more standalone and flexible - which do you prefer, will you cope with more or less support?