Eh, I get the sense that interviews are pretty rare in physics - and usually they happen after you've been accepted. Either way, if they do call you should expect it to be pretty informal. You ask them about their research, they ask you about your interests/experience/plans, you shmooze about the school/city/department, if you're feeling bold you probe them a bit about how long their students take to graduate, where their last few students are working now, and so on. No putting you on the spot with obscure technical questions, unless the interviewer's a big asshole in which case you don't want to work for him/her anyway
So it's not a big deal really. If you're admitted already you'll almost certainly be funded, so all that's riding on the interview call is TA vs. RA vs. fellowship.
"In my opinion, I mean, and I'm probably totally wrong!"