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.
If it was 5pm on a Friday night and you had 2 hours of work to complete Saturday, what would you do?
Stay and finish the work. (beware, they hold you to this)