There are only two interviews, so you don't have to worry about having to hunt them down. The first is after defeating the Tyrant Mulcimer and liberating New Ignitia. The second is after the events in the Frozen Tundra, when you return to Parum. The same interviewer will be next to the warpstone. I had at least gone to the Orphanage and activate Cadium's warpstone before I went to Parum, just in case it doesn't appear for someone else until that point.