I'm at the portion of the "Revealing the Unseen" quest (from the College of Winterhold) where I have to go to Mzulft and enter the ruins. However, when I try to interact with Gavros Plinius, the man slumped on the floor to the right when I enter the ruins, I cannot enter a conversation with him. According to the guide, as soon as I talk to him he should mutter some hint about the quest and drop a key that lets me through the door ahead.
I've tried talking to him, attacking him, pickpocketing him, and none of these things work. I've done other quests and then returned to try again, but still I'm unable to interact with him and get the key to enter the ruins. What can I do to continue this quest?