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.
Tell me about your favorite project.
I ran into my portfolio and talked about that project/process.
This is a common question so prepare thoroughly. Run through your creative process, iterations, and outcome of the project.