What to expect for an on site interview in German University?
For a Ph.D. position, they want to know:


Are you really familiar with your master thesis' subject? Can you answer questions about it? Can you explain your thoughts? This is mandatory.
Which relevant experience do you have for the institute and the future research? Some of your competitors may not have any. This is where you can enjoy the interview and get information for yourself.


So ideally you start with reporting about your previous research (your master thesis). Then you can say something like "as I wrote my thesis, I noticed that (bla bla), and as your institute does (bla bla bla), which is somehow similar, I hope to deepen (bla bla)." This would be very good. First, because it shows that you have relevant experience and second, because it helps you to check if the institute is actually relevant for you. 

Remember that a job interview's purpose is to get to know each other, it's not a one-way police interrogation. For a Ph.D. position, this is especially important because in Germany, your final failure at a Ph.D. programme would also make you inadmissible for similar programmes in Germany. So it's vital for you to sense possible (interpersonal, academic, financial) difficulties early.