Firstly, I want to point out that my opinions are based on my own experiences. Having said that a few years ago I interviewed with professors form several "top" ranked computer science, information science and business programs for potential doctoral admissions. Second, I have an admissions season's worth of experience from the "other" side where I have interviewed potential PhD applicants as well.
You do not need a research proposal. In fact, treat this "interview" as a conversation between an academic and a potential future colleague. There is nothing to "prepare" per se. Basically, what's going to happen is that the professor is going to ask you about your research interests and experiences in greater detail than what can be evidenced from your CV or publications. You will have the chance to tell him all about yourself. Do not mis-represent yourself or veer off paths you have not trodden before. What's also going to happen is that this professor will tell you about his project or his lab if he a potential adviser or person whom you want to work with during your PhD.
There is nothing ask him about preparation. Just be ready to discuss your own works and have some questions ready to ask him. Just treat it as a discussion between two professionals and you will be fine.