Even if you're not using GMC, something like character breaking points might be a good idea for staff to have, if your focus is on horror. Unknown Armies uses "stimuli" (What will make your character rage, what makes them scared, and what makes them be better than they usually are.) that serve much the same purpose.
Keep plot scenes small and intimate - the more PCs, the harder it is to hold the right atmosphere. I think some of the /best/ horror I ever ran was a solo campaign in CoC, but probably my most successful game (in terms of people being actively creeped out and talking about it years later) had four PCs, and I deliberately split them up at times, and used the MU* tools to ensure people were receiving, in some cases, personalized emits and pages that isolated them emotionally, if not "physically".
Slow burn is also a part of it, as is the unknown factor. WoD, in some ways, buffers horror by making it known - oh, these are the werewolves, these are the vampires, and the PLAYER knows what they can do, even if the character has never heard of them before. If you want to really run a horror game, throw that crap out the window, or have your monsters deliberately violate expectations. Furry murder-machine is a physical threat, sure, but it's more of an action piece than a horror. Watching your ten-year-old foster kid scream and convulse on the floor, her skin splitting open and /something awful/ clawing its way out to focus on you with three pairs of eyes the color of blood moons?
Horror tends to also be as /personal/ as possible. Even more than having your foster child turn into a monster, having someone focus on something horrible that is happening to them, bit by bit, can really bring the horror home. Describing to someone how they can /feel/ something underneath their skin, scrabbling and scratching as if looking for a weak spot to tear its way out can evoke more tension and invested feeling than describing someone else's pain or injury.
Also, I would suggest that really effective horror involves choice. There's an appeal in the "helpless" sort of horror, but if you really want players to get /involved/? I always recommend putting them in situations where they have to make choices. One of the most effective horror scenes I ever ran involved a PC having to decide to cut off his own arm...and because he was the only one of the group who had the tech knowhow, having to /talk another PC through how to cut off off his arm without killing him/. I didn't make him do that. I didn't even tell him, "If you don't get rid of that arm, it's going to kill you." I just made sure that the world around them showed the consequences enough so that when his arm was infected, he had the information he needed to make that choice and follow through on it.