I first had an on campus interview with HR which was fairly low key: they mostly asked about my previous lab experiences, skills, techniques, etc. After that, they tell you to submit some application materials: three letters of reference, official transcript, an inventory of skills checklist that you have to fill out, and a preliminary list of labs you might be interested in. Unexpectedly, I missed the deadline to submit these materials, but they still contacted me via email in early April to set up some on-site interviews. I met with three different labs in one day: most have you talk to a postdoc or research faculty under whom you would potentially work, and then they give you some time to talk to current RAs in the lab to just ask about their own experiences at the job. Pretty low key, they spend half or more of the time talking about the lab and what they research, and the other half asking about your lab experiences, interests, future path, etc.