The interview is meant to be an informal process for the mentors to better become acquainted with the students. However like all interviews, it is best to come prepared.
The mentor would have already reviewed your application, but it is always good to bring two copies of your resume: one to leave behind and another for you to look over in case it comes up in the interview.
Labs tend to be less formal than corporate settings, but that does not mean you should not dress well for the interview. Demonstrate to your interviewer that you are competent not only scientifically, but socially as well.
For more ways to prepare, here are 100 potential interview questions from Monster.com.