Fantastic, plenty of good ideas there, thanks for all the input so far -- any additional thoughts are still more than welcome!
Have a second badass looking monster, that gets to see your monster, and is absolutely terrified of it?
I don't really want to involve multiple monsters, but I'd completely forgotten the (quite obvious in retrospect) idea of having other characters interact with it; I don't want other monsters, but I can use other victims!
Maybe dust shaken from the roof too.
Have lots of physics objects (barrels, crates) in the corridors you run through, that get smashed into by the unseen monster so that they fly in front of you?
I guess these can be broadly categorised into "environmental effects" of the monster, again a good way to show what the monster can do without necessarily having to actually show it.
Camera shakes associated with the monsters footsteps?
Dolly zoom as the monster gets closer?
Interesting, I was thinking about a "blacking out" effect as the monster actually attacks to help maintain the mystery if the player dies, but I hadn't thought about other camera effects at all. Definitely great ideas for a movie, but might they break suspense in a game where you're supposed to be the character rather than watching a camera?
Speaking more in theory, there's also a difference in the fear caused by the knowledge that we have of the monster.
In some games we know what the monster is like and we know where it is.
In some games we don't know what the monster is like and we don't know where it is.
Absolutely! I'm thinking more of the second situation. It will become abundantly clear to the player that encounters with the monster are deadly and that it cannot be defeated, but I want to keep the specifics of the monster a secret so the details are left to the player's imagination, and I'd like to keep them on edge having to second guess every turn of a corner or entry into a room not knowing if the monster might be there.
Even if the player does see the monster, this does not mean that the atmosphere is ruined. For instance, you can arrange the lighting to never reveal much of the monster, or you can design the monster in such a way that it slouches and has an indiscernible form, or it has a dark colouring.
Excellent ideas. I've realised I can look to some older classic horror films for inspiration, as they often kept their monsters mostly hidden because they didn't have the realistic effects needed to make something they could show.
I'm not sure if this helps, but there's a "Developer Session" from EGX Rezzed 2014 on fear and horror in games:
I haven't watched it yet, but thanks so much for sharing this -- I'll let you know if I find it useful!
...and I've got to run for an hour or so, but I'll reply to the remaining posts shortly! Thanks again!