Allisont, I don't think you need to prepare for interview. Questions are always the same, starting with "Why us?" ending with "Do you have any question?"
They contacted me on April, 1st and we scheduled the interview for April 10th. But it may vary depending on the department. I applied for SPR program. One more note: I think I was one of the latest interviewees, because of the additional documents they demanded, since I was a non-US applicant, so they may contact you earlier.
I have heard that USC is very bureaucratic university when it comes to application process, but it is a great university (at least the grad students I had contacted told me so ).
A tip that may be of use: show your knowledge on the university, e.g. the centers of the university, its philosophy, etc. It really helps if they see that you really want to be there.
Wish you luck!