Right--my approach is to call or email the interview subject before I begin pitching with something like, "I'm preparing a pitch to xxx magazine. If I get the assignment, would you agree to an interview?" And then I go on to tell them a little about the query process (it could take several weeks before I know anything, I may pitch more than one mag, etc). I also let them know how much of their time I'll need for the interview, and ask if they'd prefer an in-person or a face-to-face interview (depending, of course, on whether or not a face-to-face is even feasible).