Q, who is oblivious to Sirius' frustration, goes on. "There's really no way for me to know who knew me or who knew some other me, given the apparent dimensional instability of this place." He frowns a bit. "I think that both Severus and Professor McGonagall would be able to tell if I started acting very strangely again though. They've both been through it before and would be instantly suspicious if I did."
"Mmmm-hmmm," Q says noncommittally, flushing. He's leery of giving Sirius anything he might use as ammunition against Severus, even if this is a kinder, gentler Sirius than the one he remembers.
Q is almost giddy. He laughs again at the look on Sirius' face. "When he possessed me, Voldemort made me stutter. He thought it would make me seem less threatening. After all, who would suspect p-p-poor, st-stuttering P-Professor Quirrell?" His laughter fades instantly. "And it worked. Harry hadn't a clue that I was the one trying to kill him."
Q is totally unaware of 'The Thing', except as a movie of course. ;)