I still think it's a bit early to hear back for formal interviews, so I wouldn't stress about them yet! Even if you have previously spoken with your prospective advisor, some schools will do a separate interview as part of the process, usually with multiple professors (for instance IFA, Columbia, Stanford, Berkeley, off the top of my head - there are others). You can look at the results board from previous years to get a sense of which schools include interviews as part of their process. I will say, it does get a bit confusing because some people post informal chats as interviews, and I certainly think there is a range between formal interviews and instances where an individual professor wants to get a better sense of a prospective student after reading their application.
When I applied last year, some professors offered to speak when I indicated my interest in applying, and then I was asked to interview formally for some programs. However, because this was my second cycle, I was personally more laissez faire and felt I could have more in-depth conversations in the spring, if I were to be admitted.
Hopefully this helps!