The final Diablo III panel of Blizzcon 2011 was all about the making of the Black Soulstone cinematic. The panel featured seven people from the cinematic team all with different job responsibilities, and it was a very informative session. You can watch the whole hour-long panel now, right on top of our Diablo Wiki article on the panel. That article has a lot more than just the video, of course. I typed it in real time, went back to edit and fix it up later, and inserted a ton of screencaps taken right from the high quality live stream.
There are 70 screenshots from the panel on that wiki article, and most of them are 1600×800 or larger, giving a far better view than you’ll get from the YT video. Lots of cool stuff was shown, including numerous new pieces of concept art for Leah, Azmodan, and numerous assorted demons.
Check it out; a few of the pics from the page are sampled below.