In preparation for an interview, come ready to discuss your portfolio at length as well as your design process and what design software you are familiar with. Graphic design positions can be very specific to the company so make sure you brush up on the company's current design style and are ready to explain how your style and experience will mesh.
I do not believe that my portfolio was fully prepared for that specific interview. There were a few gaps (such as html emails) that they were looking for, that I have, but had no examples of.
I offered to email examples after the interview to fill in the gaps.