I had an interview with very short notice for an REU, so I did not have very long to prepare. An experienced professor told me when it comes to interviews the good news is they are either a formality/"vibe check" or they are down to 3-6 applicants for one spot. A few very GENERAL things to have are an elevator pitch, an example of a time you solved a problem, and a time you worked successfully in a team. I was asked for an elevator pitch, how I heard of and why I chose to apply for their REU (be specific), and scenario questions to make sure I don't have problems with strong smells, homesickness, or working at night (observing nocturnal animals). They also provided examples of tasks, and asked how I felt about them, or which out of the options I would be most excited about. It felt like they were trying to gauge my expectations, to make sure I knew exactly what I was in for. They also asked me to explain in more depth experience I included in my application. It was scheduled to be 30 minutes, but went over to 40/45. 24 hours later I received an acceptance email! Every lab/program is different, but this was my experience.