What I noticed during a couple of my interviews last year was some interviewees constantly trying to be "impressive". PI's have already reviewed your application and already know you are qualified, now they just want to get to know YOU, as a person. Focus less on trying to be better than others or trying to prove why you deserve a spot. I recommend instead focusing more on trying to be your authentic self and everything else PsyDuck90 said. When I interviewed I made a document for each school I got an interview at and went through their website and their handbook and write down any questions I had: specific to the program, specific to the PI, specific to graduate student life, and specific to how they address social justice issues. I also added some of my key points I wanted to try and touch on during my interview with the PI (this was more to ease my nerves in case my brain went blank), such as my research interests and research ideas that could complement their line of research. I honestly had a great time during my interviews and I think a lot of it had to do with me just reminding myself that the hardest part was getting the interview, now it was more about getting to know me and about me getting to know them and how my quality of life would be if I were to get an offer. Also make friends with the other interviewees! They will eventually be your colleagues and you might see them at other interviews or future conferences.