PhD interview questions
I never had to do an interview for my PhD position in the US, but it seems like asking process questions is reasonable. Going straight from a BS to a PhD program in the US is pretty common, but you generally do most of a Master's degree on the way by taking lots of graduate-level courses. I understand that this is less common in other country's systems.You should read as much as you can about the requirements for a PhD in your target university so that you know as much as you can about the process, and then ask, as knowlgeably as you can, questions like the one you've asked here. If you've done your homework, your question will sound informed, and they can fill the gaps in your knowledge without thinking you to be too ignorant about what's allowed and what's expected of you before you join the PhD program. 

I wouldn't be too effusive about the program or professor when you are asked about your interest in the program. It might sound like you are trying to flatter your way into it which might be off putting to some interviewers. You should be fine saying that you are interested in topics like what Prof. X is working on, etc., but I wouldn't say that they're the best thing since sliced bread. Be honest without over selling it.