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.
It's rare to demand an interview. Having said that, I know that in some disciplines (parts of bio, for one), students are brought in BEFORE the final decisions to meet with faculty.
This is highly field-dependent. In some fields (e.g. psychology), almost all programs do at least a Skype interview, and the vast majority of candidates do on-site interviews (sometimes trips are paid for, sometimes they aren't). Other fields (e.g. math, econ) rarely do interviews, and instead invite students to come to a visit day once they are admitted (often paid for). If you don't know whether interviews are common in your field, I would suggest finding someone to talk about the interview process: your adviser(s), a current graduate student, or even other applicants are all valuable resources.
Almost every program that does interviews will offer some kind of remote interview for candidates who can't make it. However, I think that the majority of the time your chances of admission will be higher if you go to an on-site interview to meet people in person, though the size of that difference is debatable. Also remember that on-site interviews are valuable for you as well, they give you a chance to evaluate the program and the people there.
