When an interviewer asks about your research interests, you may be assessed for several things simultaneously. At the most basic level, it is getting to know you. What are you interested in? What do you want to research? At another level, you are being assessed for fit. Does the program have the potential to conduct the research you desire? Do your research overlap with existing lab or PI activity? If you are to fit into this lab and work with this PI 5 months from now, your interests should be feasible and at least loosely related to the focus of the lab or PI. Otherwise, why are applying to work with this individual or lab? Finally, this question can also be used to assess your scientific thinking. I have heard of some (rare) professors who prefer individuals to answer questions like "If you had to pick today, what would be the topic of your masters thesis?" or "If you had unlimited resources what research would you conduct?"
I hope that my response does not scare any potential applicants, but I do want to inform you. This question is not necessarily a free for all "tell me what interests you" question. It is a way to assess fit. If you want to research XYZ with expensive equipment and the program researches W with paper and pencils you may not come across as having a "fit" with that lab. These are all my own opinions and experiences, so take them for what they are worth (probably very little).