Granted, you have more than 30-seconds to make an impression in an interview. In fact, if you've come this far, your work speaks for itself. Usually, agencies won't call people in for interviews unless they really like the work they've seen.
Here are a few tips to help you through that chemistry test:
1. When someone is looking at your book, resist the urge to talk. No matter how uncomfortable the silence, don't try to explain your work. It sounds Full Metal Jacket-ish, but when someone's looking at your book, don't speak unless spoken to. From the other side of the table, it's kind of annoying to try to focus on work when your mind is constantly interrupted by comments like, "Yeah, my professors really liked that one."
2. Rule of thumb: Ask more questions than you make statements. Ask about the agency. About why your interviewer took the job there. The best part and the worst part about the agency.
3. Listen. I can't remember who said it (probably Stephen Covey), but the problem with most of us is that we aren't really listening when the other person is talking. We're trying to figure out what we're going to say next. It's even more complicated in an interview because we're trying to figure out what we're going to say next that makes us sound so smart they'll hire us on the spot. Step back. And really try listening. Then answer as best you can. Keep in mind, they probably already like your work. So you've got that going for you.