Many, if not most, interviews end with the interviewer asking the interviewee if he/she has any questions. You need to prepare for this. The interviewer is not just being polite. How you respond to this question is a good indicator of the kind of employee you'll be.
A response of "No, you've pretty much covered everything" is an extremely weak answer. Don't go there if at all possible. Instead, take a look at the articles listed below and have some questions for the interviewer in your back pocket, in addition to any questions that occur to you during the interview. There are, of course, many more sites available on the Internet that give this kind of advice, so feel free to do your own web search as well.
Always go into an interview knowing that you'll be asked if you have questions and knowing what you're going to ask. Asking questions shows that you're interested in the job and the company, that you've done some homework, and given some thought to the position and what you're looking for in a job.