Second interviews are required by employers who use the first interview as more of an introduction to see if you will fit as part of their team. The second interview, then, will probably be more in-depth. Be yourself, your best self, and you will ultimately have a positive experience. The best thing about second interviews is that it usually provides you the opportunity to view the organization in action and to meet some of the people with whom you will be working. Remember, you are trying to decide an important part of your future.
Observation can provide a large part of the answer.