Research
Let's face it; your average employer will interview multiple people for a single job. At least one other person besides you is likely to have the basic fundamentals covered. That is, they will be reasonably qualified for the job, presentable in appearance, and sociable enough to navigate through the interview without serious mishaps.The real trick is in distinguishing yourself by appearing more acceptable for the job than absolutely necessary. One of the best ways to do this is to first do some research, and then ask questions during the interview.
First, research the company beforehand if you are not employed there. Don't get caught looking silly by showing ignorance about what the company does, or how it is positioned among its peers. For this, the first place to check might be the company's own website. While setting up the interview time, it is also considered appropriate in most fields to ask for corporate literature or information.
If the business is local and public, consider stopping by before the interview. You may be able to walk around the establishment or take a guided tour. If you are applying at a large company far away, you may consider checking financial news sources for information on the company. Both the Internet and library searches of newspapers may be helpful. The amount of research you do will understandably depend on the job and salary you hope to get.
Also, find out as much as possible about the job. You may be able to get some information while setting up the interview. If you are seeking a promotion within your own company, don't hesitate to start a casual conversation with someone in the department the job is in. They may have inside information on why the job is vacant, or what exactly the interviewer is looking for.
While being interviewed, you have the option of either demonstrating your knowledge or keeping your fact-finding a secret. In many cases, interviewers will be impressed if you know pertinent facts about their company. In other cases, they may be surprised if you specifically mention personal traits that match their needs when they have not told you what traits they are looking for. Don't be afraid to ask for clarification if you are asked an unclear question. In some cases, your question may demonstrate more knowledge than an obscure answer would have.
After the interviewer has finished with their questions, it is your turn to ask what your duties will be in the new post, what the starting salary will be, whether there will be opportunities for advancements and raises, and so on. If the job is technical in nature, ask a technical question or two as appropriate. This will make the interviewer feel that you are actively interested in the position, and will reinforce the feeling that you are very knowledgeable in your field. Do not ask more than a few technical questions, or the interviewer may start feeling annoyed or defensive.
Asking questions during the interview also subtly raises the interview relationship from one where you alone are being judged, to one where you are on an equal footing with the interviewer. While this is purely psychological, it is a powerful tool in interviewing. You may find the starting salary has been raised for you, or you were considered more qualified than the other applicants, purely on the basis of the interviewer's inner feelings that you are closer to being their equal.