I will focus my advice on preparing a horror game for a Cypher System game, but the advice applies across a variety of systems. For a detailed discussion of Cypher System mechanics to facilitate horror stories, check out the Cypher System Rulebook. Instead of mechanics, I want to provide some simple tips to help with adventure design and session management.
|Horror by W H (flikr)|
1] Understand your game -- It essential that you understand the sort of game that your system facilitates. The Cypher System (unless modified greatly with the horror rules) creates broadly competent characters. This is similar to the Fate system and other recent game systems that were built to create a high action / adventure game experience for players. In these systems, it may be difficult to create genuine fear that the characters are in danger. This will likely require the use of special rules to increase danger or a dedicated attempt to create more of a threat than usual. A true horror experience is not the intent of these systems "out of the box" so recognize that you will have some work to do to make the systems provide a horror game experience.
2] Horror and humor -- One of the dangers of using a high adventure / pulp action system for a horror game is that the tone of the game will gravitate towards humor instead of horror. With highly competent actors lacking clear vulnerabilities, a horror game can quickly veer more towards Goosebumps than Joe Hill. To some degree, this is not something you can or should fight. It is natural to have a little humor in the game for contrast. Most of the people with whom I have played have wanted humor to be part of the game and crushing it whenever it emerges will just lead to frustration. Instead, carefully consider your various scenes and challenges. If you want to de-emphasize humor, try to avoid elements that will likely lead to a humor dominating the tone. Games like Numenera makes this a little easier (though this can work in any setting) by providing opportunities for weird elements to bridge between humor and horror.
3] Changing gears -- What do you do when you want to re-establish a horror tone? The key is creating a clear break between scenes to facilitate a change in tone. You should pull all of the performative tricks out of your GM tool kit. Allow a long silence to create a sense of expectation that something is about to happen. Switch the pace of your speech to indicate the change in tone. Change your diction -- possibly moving from quick, simple descriptions to deeper description to indicate the importance of what you are describing. Naturally, you will want to include language that suggests the horror you want to convey. Describe the environment in a way that conveys how the characters are experiencing the world around them. If the trees hang over the characters oppressively and the shadows are dark yet seem to suggest movement -- the players will sense what the tone is.
4] Have reasonable (and shared) expectations -- Horror gaming is difficult. I don't mean this just in the sense that Call of Cthulhu characters have a notorious fatality rate. It is hard to maintain the tone of horror among the players. In my experience, games tend to be happy and joyous times. Friends are getting back together with the intent of playing a game. Having them switch their mood to fear is difficult. For this reason, you need to have reasonable expectations. It is not going to be a four hour segment of people twitching nervously until they all scream in unison at the climax. There will likely be a lot of smiling and joking. Pick your moments where you want to emphasize horror and box those scenes off with clear markers or the tone.
Maybe most importantly, horror gaming is precarious. There is no real threat to the players (one hopes). All it takes is a single joke to break the mood. If the players are not invested in having a horror experience, each one will be able to break the mood at will. Instead, the group needs to discuss the intention of fostering a horror tone, acknowledge the limitations of the horror scenes (including discussing the limits that the players want to the content of horror scenes -- possibly with a mechanism like the X card), and agree on the signals that indicate that a scene is going to focus on horror. With shared expectations, everyone will find it easier to work within the horror tone during the targeted scenes.