|Winona Ryder steals the show, lemme tell ya.|
Joyce Byers is the perfect NPC for Call of Cthulhu.
She's perfect for any horror game, really - anything that will involve dealing first-hand with the dangerous, horrible unknown - and the why of that comes down to a pair of traits.
- Joyce is aware of what's going on, but doesn't necessarily know what's going on.
- Joyce has the ability to deal with what's going on (even when under incredible mental strain), but doesn't have the vocabulary to express it.
This puts her in the unfortunate middle-ground between the mundane and the supernatural worlds. She can handle the situation to a point, but hits a wall. Her methods are stopgaps that she can't properly explain, and she doesn't have the time or resources to do anything more complicated or thought-out. We the audience can see the logic behind what she's doing, but the other characters see it as madness. She's not crazy, just correct enough. With her limited knowledge and likewise limited ability to expand that knowledge (barring aid from others), correct enough might not cut it.
This is the sweet spot.
You don't just have to use Joyce in the typical manner of investigative lead, exposition, or warning: you can use her to think of how your players are going to interact with the horrible mystery presented to them, and how you are going to have the mystery interact with them.
How much knowledge will they gain? How much ability do they have and how much will they end with? Will they gain vocabulary to describe things? How will any of these traits change over time, and how will they change? Will knowledge be proven to be false, vocabulary revealed to be inadequate? How much damage can the difficulties in communication cause? How much can "correct enough" protect you? What are the consequences of Joyce's incomplete knowledge? What are the consequences of the players' incomplete knowledge?
Here's a simple example to end things off:
The players meet with Joyce, who has taken to keeping her room boarded up and fully illuminated through the night. This is to keep the monster away. It has been thus far been effective, and her run-ins with the monster have ceased, for the present.
This is not because the monster is afraid of light; Keeping the room in complete darkness will also work. The monster is attracted to lights within a greater darkness (a lamp, a torch, etc) in a way something akin to a deep-sea fish. So, the player characters, working off of technically correct but incomplete information, go out hunting the monster, thinking their lights will help them. It doesn't go well, but those that manage to survive have a new piece of information for their investigation. Joyce, seeing how their deviation from her method ended in disaster, will likely continue on her path, and evidence from elsewhere will have to be brought in.