[Update: and now there's a fifth discussion thread, which you should probably use in preference to this one. Later update: and a sixth -- in the discussion section, which is where these threads are living for now on. Also: tag for HP threads in the main section, and tag for HP threads in the discussion section.]
The third discussion thread is above 500 comments now, just like the others, so it's time for a new one. Predecessors: one, two, three. For anyone who's been on Mars and doesn't know what this is about: it's Eliezer's remarkable Harry Potter fanfic.
Spoiler warning and helpful suggestion (copied from those in the earlier threads):
Spoiler Warning: this thread contains unrot13'd spoilers for Harry Potter and the Methods of Rationality up to the current chapter and for the original Harry Potter series. Please continue to use rot13 for spoilers to other works of fiction, or if you have insider knowledge of future chapters of Harry Potter and the Methods of Rationality.
A suggestion: mention at the top of your comment which chapter you're commenting on, or what chapter you're up to, so that people can understand the context of your comment even after more chapters have been posted. This can also help people avoid reading spoilers for a new chapter before they realize that there is a new chapter.
Has Quirrell been kissed by a Dementor? With Voldemort responsible, presumably.
That would explain his zombie mode - when he slouches, drools, doesn't speak, and can only stagger around.
And in chp 45 the Dementor could have been speaking to (zombified) Quirrell rather than to Voldemort when it said to Quirrelmort "that it knew me, and that it would hunt me down someday, wherever I tried to hide."
Voldemort can take over Quirrell and act through him, turning him into articulate Quirrell. But when he's not actively in control Quirrell enters zombie mode. Voldemort might be going someplace else while he leaves Quirrell on autopilot, or maybe he just needs to rest because controlling Quirrell uses up his energy.
It can't be this; a Dementor-kissed body is a vegetable which doesn't do anything at all, much less shuffle around slowly. (Checking the HP wiki, it seems a kissed-body does still breath and have a heartbeat.) If Voldemort is controlling the body, he has to be controlling it at all times in some capacity.