Answers that show you're unprepared:
- That's a good question, I never thought about that.
- I'm not sure, I liked them all.
- Oh, I don't know, I think you've covered everything.
Many times I'll ask a question, not so much because I want to hear what the answer is, but rather, I want you to talk and interact so I get a sense of what kind of person you are. How you think.
Rehearse before you get to the interview.
By reviewing common questions at interviews and preparing short but interesting answers of two or three sentences each, you help to give the interviewer a lot more insight into you as a person.
Prepare some questions of your own.
Asking the interviewer something besides, "how many weeks of vacation" or "what are the hours" is more impressive. Insightful questions help to round out the impression left at your interview.
I like to start the interview with some small talk just to loosen up. Something about the weather or directions "did you find it okay?" doesn't require a long involved answer, but again, a few sentences to communicate.