I too love the interviews that sometime follow, or occasionally, begin gokkun videos. Just did a quick, non-exhaustive review of some titles that I remembered having them. I think interviews were usually found on DVD/VCD releases. Years ago, many videos I had were DVD rips, but over time I switched to other versions, either out of necessity or due to improved quality, so I lost some interviews that I thought I had.
These ASFUR titles have pre or post interviews and/or BTS content:
Ai Uehara ASW-191
Anju Natsuki ASW-116
Arisawa Hazuki (saaya hazuki) ASW-047
Arisawa Hazuki (saaya hazuki) ASW-028
Erika Hiramatsu ASW-071
Kokoro Tsukino ASW-185
Mai Tamaki ASW-190
Maki Hoshikawa ASW-187
Maki Hoshikawa ASW-223
Mika Osawa ASW-051
Miki Kitazaki ASW-006
Mina Hirayama ASW-044
Mirei Yokoyama ASW-059
Nana Kunimi ASW-053
Reika Kudo ASW-009
Reina Akitsuki ASW-052
Remi Sasaki (ren mukai) ASW-156
Rena Konishi ASW-108
Ryoko Murakami ASW-029
Sayo Arimoto ASW-158
Yukino Hikaru ASW-017
Yuri Honma ASW-247
Yurina Ayashiro ASW-162
There are fun interviews in a couple Junk Shop titles as well, I believe only found in the DVD rip versions out there. Ayu Sakurai and Hibiki Otsuki (separately) sit with the director Rusher Miyoshi looking at their own videos as well as gokkun titles from Miyoshi's past.
Ayu Sakura NS-11
Hibiki Otsuki NS-10
And finally, there is some bonus footage associated with Kirari Koizumi's PSD-069 that is almost entirely interview and BTS content.
Enjoy