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.
Explain your creative process.
That is difficult to quantify since inspiration can occur anywhere or at anytime, sometimes related to the problem at hand, sometimes not. It's a spark that leads down a path that isn't always clear until the end has been reached.