I was interviewed by 2 Principals (one was over my studio who would actually be hiring me, the other other was just there to be an extra ear and mouthpiece). The interview probably lasted about 30-45 minutes. Definitely make sure you have an up to date portfolio on hand (and can explain the projects), as well as copies of your resume for your own reference and in case they don't have one on hand. It shows that you came prepared. Also research the company ahead of time and have some questions on hand that you would like to ask. Try to make them specific to the company so that they know you did your research. It shows interest. Make sure you don't ask a question that was already answered previously. Gauge what questions to ask based on the tone and direction of the interview. I was told in the interview that you're expected to work a certain number of hours per week (over the general 40). It is good to know this and be okay with this upfront.