Your interview actually starts when a representative from your potential employer calls you to make an appointment. And it only ends when you are officially employed or rejected. So, it is important for you to be on your best behaviour at all times.
You can easily lose the opportunity if there is negative feedback from the person who arranged your interview. Every person from the company with whom you have the slightest contact can affect your chances in ways you can't imagine.
Don't take any chances. Be prepared.