Weird
 

I was in a group that killed Diablo and then moved to Act V. The rest of the group went and did Shenk immediately while I stayed and watched the video between the acts. When it got done and I entered Act V they had just finished the quest. Odd thing was, I got credit for the quest as well even though I had been watching the video and not done a single thing to help. There was a free socket quest for watching the video. Never seen that before..

when a player that doesnt have the quest is there to complete it all players in town get the quest too. When ur watching the movie u actually allready are in town, and they completed the quest so u got it too.