My best advice honestly would be to quit preparing. Just go in there and be you man. Stressing over it and trying to prepare perfect answers will just worry you more and make you nervous. When you interview, you just want to be calm and cool, and be yourself. Give honest answers. All you've got to know is who you are and what you'be done, because that's all they want to know, and everyone knows about themselves. Every interview is different and impromptu usually, because they interview goes on the path that you take it. The interviewer asks questions about you, so you choose that path. Sure they might have questions they pool from, but if you answer "I've never had any work experience" they won't ask you "So tell me all about your last job," so really you can't really prepare for that except knowing yourself. So be you. If you want to prepare, then learn stuff about the company. Learn their website, what they stand for, stuff like that.

Another thing, to make sure they see you are interested, is to view it like you are interviewing them as well. This interview is likely the last chance you have before getting the job, just as it is for them hiring you. So you gotta make sure this what you want. Ask them questions back, information about their company you couldn't find or understand. Don't ask about pay, not for a job like this. But general info stuff, like maybe about what you'll be doing, the work environment, etc.

Hope this helped, GL. I'm pretty much a king at impromptu so for interviews I almost never prepare. But eh, not everyone's like me. Still, hope this helped.