All of the above is excellent advise. Kind of hard to add anything to it, but I'll share a couple of things, which during my internship earned me a lot of credibility with the students: I put myself in the student's wetsuit boots and remembered how I felt when I was a student (sort of scared yet excited at the same time), then I kept a watchful eye for those silent reactions on the students and just offered support. It helped to be in sync with my instructor at all times, and anticipate what was going to be his next move, which helped the flow of the class.
Another thing, and most important of all, remember you're there first as a student yourself, then as a "divemaster;" wait for your instructor to ask for your assistance, whether it is to turn off the student's air when simulating an OOA situation, or it is to demonstrate a skill, UNLESS you see a student is in distress and you can get to him/her before the instructor.
Otherwise, just enjoy yourself. This will be more of a learning experience for you than it will be for the students; you'll not only be reviewing your own skills, but also learning how to properly demonstrate them and to recognize students' behavior. Be a support and a leader at the same time.
Good luck, and let us know how it turns out!!!!!