I'm paranoid, sleeping with my finger on the trigger. (Geto Boys, Mind Playin Tricks on Me)
Here's an idea I had for Call of Cthulhu. I have yet to playtest it, so take it with a grain of salt, if you don't already do so with everything I put on this blog.
There's a weird nexus in RPGs called "playing your character", and it comprises both "acting out your character" and "seeing things through your character's eyes". The former is where you get stuff like not doing things based on information your character doesn't know, or speaking with a funny accent, or anything that displays the character to an outside observer. The latter is where you get excited by success and troubled by failures, as if it were really you going through the travails of your character.
Call of Cthulhu treats this nexus a little differently than most other RPGs. You're supposed to enjoy the game regardless of whether it ends well for your character or not--and it often ends badly for your character. You know this going in; it's part of the fun of the game. But as a side-effect, it's hard not to treat your character with a bit of ironic distance. You see the game less like Night of the Living Dead and more like Shaun of the Dead. That distance makes it hard to see things through your character's eyes.
"Acting out your character", on the other hand, has a great hand up because of the Sanity rules; seeing things Beyond Mortal Ken causes the stat to drop, and dropping too much too fast will cause him/her to run shrieking in fear or collapse in a catatonic heap, or be bedeviled by any number of phobias or delusions.
Great for presentation. Not so spooky to experience.
So there's got to be something that the Keeper should be able to do to make the player feel a little bit of that fear, revulsion, and paranoia that the character is supposed to be feeling. And I think the best thing to do is rip off an innovation from this game.
Eternal Darkness: Sanity's Requiem is a horror video game with a look adapted from Lovecraft and mechanics clearly derived from Chaosium's RPG. Specifically, your character (whichever one you're playing at the time) has a Sanity bar that goes up and down during the game. But rather than force your character to run or break down, Darkness plays tricks on you when their sanity drops below a certain point: statuary seems to be turning to look at you; your character starts sinking into the floor, as if it were made of molasses; casting spells causes them to explode, then come back and realize that it was just a hallucination; and at one point, the game pretends you've hit the Blue Screen of Death.
The trick here is, the game is presenting you with this information deadpan. It's a lie, and you have to see through it to get past it.
So what I'm suggesting is that you, as a Keeper, lie to your players. Actually, what you'll need to do is present them with multiple, competing versions of the facts, and let them decide which one to believe. Say your characters encounter a shady-looking NPC. He could be a spawn of the Deep Ones, or maybe it's someone else. You present your players with two index cards:
- On one, you write: "He seems to be normal, maybe a little bit scared of something himself."
- On the other, you write: "He seems evasive, but as he turns his head for a second, you think you spy gill slits on his neck."
So the players have to decide whom to believe. Now you might ask, when do you give these out, and which player gets the "true" card? Probably the easiest thing to do is determine beforehand what facts can be played as hallucinations and just pass them out then. In the case above, it's when they first meet our NPC. But it could also be at any point after someone fails a Sanity check: suppose you're running away from a shoggoth, and you enter a room with a cave exit on the right. Time to pass out the cards, but one of them says the exit's on the left. And running into a cave wall when you thought you were escaping can put a real damper on your day...
But who gets the "real" card? It's too easy to say that the person who's just lost SAN gets the hallucination card, because all of the players will know not to trust him. Better to have a roll-off between any and all of the characters who have ever lost SAN (which should be everyone after a few sessions). (Alternately, anyone who has ever had to make a Sanity check, regardless of whether they lost any; your call.) Roll d100, and see how it stacks against the character's Sanity. Anyone who rolls over their SAN is a candidate for the deceptive card, even if they aren't the one who just lost it. In the case where they both make the secondary roll, you can go with the default assumption that the one who just lost SAN is hallucinating. (If you're rolling at a predetermined spot, like the Deep One NPC above, don't play this trick if everyone makes the under-their-SAN roll.)
So if Harley has a SAN of 30 and just lost 5 points, but you rolled a 23, say, he'll be given the "real" card, even if Mordecai has a SAN of 70 (if you roll over 70 for him.) You do these rolls in secret, of course. Now, the players may know it's likely that Mordecai is the one who really knows the way out, but it's not guaranteed. The trick is in the uncertainty and inconsistency.
This may be a little difficult to run with several PCs, but CoC tends to have smaller parties than adventure RPGs. Also, your players may balk at being deliberately deceived (as opposed to merely failing perception-based die rolls). But if they're game for it, give it a try, maybe on a one-shot. Let me know how it turns out!