In PPro, if you adjust volume for the Master track and set it at the proper level for the combined soundtrack/VO, then it is likely your interview track (which I assume is voice-only) will be mixed down at too low a volume.
You can either animate (keyframe) the volume changes on the Master track to bring up the interview volume, or you can create a new submix track and route the soundtrack and VO tracks to the submix, then adjust the volume of the submix to the proper levels. If you use the submix track, leave the Master track volume alone.
FWIW, for output to DVD, I set my peaks at -6 dB in PPro.
-Jeff
The Focal Easy Guide to Adobe Encore DVD 2.0