This can be really frustrating, especially when you put the effort into preparing a list of really thoughtful questions.

You can start by saying that you did have a list of questions, but that the interviewer did such a thorough job s/he answered them already. But if you use this, you need to follow up with a good question, otherwise it may just look like you're trying to ingratiate yourself with praise.

Rather than preparing a list of specific questions, you could think of general areas of inquiry, e.g., best practice or continuing education.

Or, if you really can't think of anything new, you could ask them to elaborate on something that was mentioned during the interview. For example, if s/he mentions continuing education, you could ask about how often it's offered, specialty areas that are focused on, etc. The benefit of doing this is that you not only show you are interested in the position, but that you were also listening to the information being provided.