How common are PhD interviews in the U.S.? Q: How common is it for a faculty/school to request/demand the good PhD applicants to come in for an interview?

For someone that lives outside of the U.S. and wants to apply to do a doctorate in the U.S., should that person temporarily relocate to the U.S. (at the correct time and for the correct duration of course) with the expectation that they'll have to interview in order to get in?

Do top schools vary from mid-ranked schools when it comes to interview frequency? Does it vary by faculty?
In my experience, it's typical for a US university to invite PhD applicants for a visit only after they have been accepted for admission. The purpose of this visit is primarily for the school to make a good impression on the student and try to convince the student to accept their offer of admission. An interview is not normally a required part of the application process.

However, one of the best things you can do to increase your chances of admission to a particular graduate school is having a professor there who wants you to work with them, and who will lobby the admissions committee on your behalf. In order to get this kind of connection with a professor, you will need to meet them and establish a rapport well in advance of applying to the university, and it's possible that the professor will want to meet you in person as part of that process.