It's important to prepare questions for the interviewer(s) before you go into an interview. Believe it or not, the questions you ask can actually help the interviewer answer the three main questions they have about you as a candidate. By asking good questions, you demonstrate that you've given a lot of thought to what the job will be like and your ability to do it well. Also, most interviewers are working off a standard sheet of questions they're asking everyone, and the discussions that come about because of the questions you ask can help you to stand out as a memorable candidate.
I suggest preparing a minimum of five questions to ask during the interview. You want the interview to be a back-and-forth conversation as much as possible, which may mean you find appropriate times to raise your questions in the middle of the interview. But there will almost always be a time at the end when the interviewer says, "Do you have any questions for me?" and you don't want to have used up all your questions at that point.
Here are some things to keep in mind when preparing questions to ask:
Do ask supervisors — especially if they'll be your supervisors — what employees like most and least about working there. I like this question better than asking, "What do you like most about working here?" because it gives you a sense of how in touch the managers are with their staff. I once had someone defer that question to their subordinates at the table, and later discovered this was because they really had no idea what their employees did all day or whether they were happy in their jobs.
Don't ask about salary or benefits at this stage. When you get an offer, you can clarify (and negotiate) as much as you need to about health insurance, vacation time, or other benefits that are of particular concern to you. If you ask these kinds of questions (or worse, start making demands) before you've been made an offer, you could come across as greedy or overly concerned with what you'll get out of the job.
Do make a note of each person's area of work if you're being interviewed by multiple people. If possible, ask each person a question specific to their area. Not only will this help you be a memorable candidate for every person in the room, but it will also give you information unique to each person to write in your individual thank-you notes.
Don't ask if you'll be able to get time off for prior commitments. Again, once you have an offer, you can negotiate vacation time or a later start date, but wait until you have that leverage.
Do ask what will be the biggest challenge you'll likely face in the position. This not only gives you some insights into what current employees may be struggling with, but it also gives you a clue into where they perceive your weaknesses to be, giving you a chance to respond. I suggest asking about their hesitations in hiring you for the same reason.
Don't ask questions that you could have easily found the answers to on your own. If you ask questions that could be answered by the job description or by a basic review of the company's website, you will not appear to have adequately prepared for the interview.
Do ask questions that show off your research. If you are aware of recent changes in the company or in the industry, ask what effect those are likely to have on your work. If there are new initiatives being formed, ask if you will have the opportunity to be part of them.
Don't bring up negative information about the company unless it represents a specific, immediate concern to you. Bringing up bad press the company has gotten or negative reviews you saw from employees has the potential to stir up negative emotions, which will then be associated with you and your interview. If at all possible, reframe the information into a more neutral question — for example, if you've seen negative information about company management, ask specific questions about management style and philosophy.
What questions have you asked in an interview that have generated a positive response, and what questions do you wish you'd never asked? Leave your experiences in comments!