Research is very important, but few people do sufficient.There is so much that can be done and the last thing you want is to be caught out by a question, the answer to which could easily have been covered by your own research.
Start by finding out as much as possible about the company or organisation. Most will have a website, but if not you can Google the name and see what comes up. Do you have any friends who might be able to tell you something, or maybe you know an employee? Find out as much as possible.
Understand the industry in which the employer competes. Of course, this might not always apply, but you need to know its competitors and be able, if necessary, to offer your thoughts about these competitors.
What about the industry itself? Are there any trends occurring? Is there any government legislation on the horizon (for instance, in relation to global warming policies) which might have an impact on your potential employer?
Believe it or not, you might even be able to find out something about the interviewer. Profiles of senior employees are often posted on websites. Your interviewer may well have something in common with you, for instance sport, and it's a very good ice breaker, and often creates a good impression, if you can make a remark about this common interest at the point of meeting. It can also help both sides to relax by discussing a mutually interesting topic prior to getting stuck into the 'proper' interview.
Job advertisements can tell you little about the job itself. Every job advertised is always an 'exciting opportunity'. Can you obtain a copy of the job description in advance? Ask if one can be emailed to you.
Is there anything else which is important? Think carefully. I was once interviewed for a job for which I was highly suited. In fact I was offered the position. However, I turned it down because of the low salary. I could have saved myself, and the company involved, a lot of time and trouble had I made a simple phone call enquiring about the salary range. Bear this in mind if the range is not mentioned in the job advertisement.