Companies who do on-campus interviews fall into two categories: Those who have specific positions open and are looking to fill them, or those who are merely collecting more resumes. Unfortunately, it is often impossible to find out before the interview which category the company you are interviewing with falls into.
Therefore, you need to find out which category the company you are talking to falls in. This can be accomplished easily by asking the interviewer whether there are specific positions the company is trying to fill. After all, if all the company want out of the recruiting trip is some nice resumes, there is no need for the interviewee to sweat too much during the interview. The general rule of thumb is that the bigger the company is, the more likely the company is simply looking for resumes to put into its database.