How often are there interviews? And other generic questions

[Kijoon]

Hi, I'm an Asian student applying for PhD and this interview thing is a little beyond me. 

I would like to ask: how often would a school ask for an interview? Are interviews the norm or do sometimes schools skip them?

I assume that if I do get interviews they would be online as I'm living from the other side of the planet, does anyone have experience with online interviews? Any tips?

Also what should I prepare for my interview? Would the school ask mostly of my prospective studies (meaning I should look though the faculty and their research topics) or just to understand me as a person, or both?

My field is public administration, and I'm applying for US schools.

 

Thanks.