I'd like to make a poll: what's the immediate sensation, think, reasoning, etc. you think when suddenly you encounter a Doom monster?
Do you always think the same thing when you find a certain kind of monster? And so, behave consequently?
For example, when a Revenant discovers my presence and start to follow me, I think: "Oh shit! Now I gotta run as far as possible from that fuckin' skeleton! Or maybe.. YEAH, I could try to give him a couple of double-barrel shoots if I'm close enough!" And I suddenly feel caught by anxiety, because of the psychological effect..
And which monster most make you anxious when you encounter it? By my point of view, the Revenant is the most stressing opponent to face, despite there are more enduring and powerful monster, such the Cyberdemon.

