Your preparation should include at least 5 questions you can ask them when the inevitable, "do you have any questions for us?" is asked. Having nothing in mind can be a fatal mistake.
It is a valuable chance to demonstrate your level of interest in the role, your knowledge of the organisation, and most importantly creates an additional opportunity for you to assess if this is really the role you want.
Assuming you already have a fair amount of information about the position you are being considered for, including why it is vacant, who it is reporting to, what the KPIs are, and what type of person they are looking for, the questions you ask will be delving deeper into the broader organisation.
Often, the impression you leave on the interviewer will be a result of the questions you ask them, rather than how articulately you respond to their questions. And you may find the answers they provide to your questions impact your final decision more than any other aspect of the interview process.