During the interview for a typical MD/PhD program the main focus will be on research. The interview committee will try to hit you with questions regarding the research you performed during your undergraduate years (and/or graduate if you have a Masters degree).
Unlike an interview for an MD program, which focuses on questions regarding possible commitment towards clinical health care, MD/PhD programs are almost exclusively concerned in your attitude towards biomedical research.
Interview questions therefore will revolve around what you have provided on your AMCAS and secondary applications regarding your past research experience. For example, if you have worked in a neuroscience laboratory during your junior year in college and the committee received a letter of recommendation from your research supervisor/professor, the committee could have you elaborate on what you did and how you felt about your research experience.
Be prepared to answer questions about your own research experience.