Research any recent news articles about the company. Talk to anyone you know that currently works for this company. Find out as much as you can about the work environment, company culture, benefits, successes, failures, and recent initiatives. Your research will identify both facts and opinions. Facts are facts.
Opinions require further research.
Know the Requirements of the Job for which you are interviewing. Often, people with the same job title at different companies are called upon to do different things. For example, project managers at some companies just manage the technical aspects of a project, while at other companies, the project manager is responsible for hiring, firing, the project budget, and the financial justification of project changes. It is also not enough to just know the requirements of this particular job. You must be able to site examples from your experience to show that you are capable and qualified to meet these requirements.