Les vampires : Breaking the Fourth Wall

Mazamette looked directly down-camera and reacted towards the viewer in almost every scene. Was this just a quirk of the actor - Or was there significance to this within the film? Because it was notable that he was the only character doing it.

I wondered the same thing. I don't think it was intentionally breaking the fourth wall. I haven nothing to back this up with, but it seemed like he would catch himself looking at the camera sometimes and look away.

It definitely seemed intentional to me. He does it frequently, and usually not that briefly, and with a very pointed look on his face. I think it's a pretty clear example of what TV Tropes would call an "aside glance".

Don't listen to the negative ones; their arguments are irrational.