There are atmospheric things you can do to contribute to a horrific roleplaying game. It is perfectly possible to have a very frightening game in a brightly-lit room (at a recent game everyone was so engrossed in the game that when the sun went down they totally forgot their plan to turn down the lights and light the candles). Instead, however, your group might find any of the following techniques useful. Even if you don’t play horror RPGs you might get some ideas here; many types of games benefit from a little atmosphere.
Start slow with mood techniques. One or two at a time is usually plenty. If you go wild hanging up dark cloths, turning out the lights, lighting candles, playing music, and everything else at once, things could seem more funny than creepy to your players. Less is more, and all that.
Draw the curtains. If the room has plain plastic white shades or Venetian blinds, then cover them with cheap dark cloth. You can find what you need in the sale bins at your local sewing shop, or you can use old sheets or buy a cheap dark sheet set.
If you hang up cloth or other material, make sure that the owner of your venue doesn’t object to the use of pushpins (or whatever you use). If your cloth is dusty then make sure none of your players are allergic to dust mites (it’s the little considerations that count).
Turn down the lights if you have a light with a dimmer switch (halogen lights can be great for this), or just use small dim lamps instead of bright overhead lights.
Make sure any hot halogen lights are nowhere near the excess cloth. Never hang cloth over a halogen light to color the light; you risk setting a fire.
Start your game after sunset, or just as the sun is going down. If you have a good sense of timing, arrange things so that the sun is going down just as things start getting creepy. Make sure your players are capable of staying awake past 9 or 10 PM if you’re going to do this. If they aren’t, or if this isn’t feasible for other reasons, you might substitute with step #1 (drawing the curtains or covering the windows). If you live in the country and the weather is good, you might open the windows so you can feel the night breeze and hear the crickets.
Light candles. Flickering, warm, uncertain candlelight is one of the greatest mood-setters. Keep scissors around to trim the wicks when they start to smoke. If your players have a tendency to ignore the game and play with the candles, then either put the candles out of reach or don’t use them.
Put candles in stable places where they won’t fall over and won’t catch any nearby drapes or books on fire. Preferably use wide candles, not tapers, as they’re less likely to fall over — nothing breaks up a good game like a raging fire! Make sure there are plates or other such holders under the candles to catch dripping wax (you don’t want to have to replace a wax-coated rug or table). Finally, just because it’s a good idea, make sure there’s a fire extinguisher around, and keep one lamp lit somewhere nearby so no one falls over anything if the candles go out.
Use dark-colored candles. If you make your own or buy ones with flat sides, you can carve and paint the outlines of strange symbols into them. (I find a metal tool called a burnisher, with a rounded tip, available in art supply stores, to be perfect for this task. Use it together with a few plastic design templates or your own designs.) Simple line art and unusual borders are often best. (See the above cautionary notes on the use of candles.)
Hang the room with dark or exotic cloths, odd posters, or unusual pictures, photos, and tapestries. (See above cautionary note on the hanging of cloth.)
Keep a few odd, unique, beautiful props around. Find interesting things in antique and second-hand stores. Beautiful and exotic glass bottles, antiqued mirrors, and little fetishes. These work either as scenery to help lend an exotic air to the room, or as representations of actual items within the game.
Play quiet, creepy music in the background. Preferably avoid anything with lyrics, as this may distract players who have a tendency to sing along or wonder what the lyrics are. Movie scores often work well. Music works very well for some groups and very poorly for others; this is definitely an issue of personal taste. If it doesn’t work for you, don’t use it. If you have a particularly good sense of timing or you know the CD very well and don’t mind mucking with track order while you’re GMing, you may be able to use the music creatively to enhance the buildup of a creepy feeling. If your timing sucks, you may instead end up with the love theme playing during the climactic battle. Keep in mind that if you let the same CD run over and over for an entire evening, you may drive some players stark raving mad.
Music is one technique that ports particularly well to other types of games, as you can always find music to suit whatever mood you’re going for.
Have your run in a location that promotes atmosphere. This may be a dark basement or musty attic. Some groups even have their games in really odd outdoor places, but this may cause more problems than it solves. Make sure none of your players are allergic to musty places or whatever plants are in bloom before you try this, and make sure you’ll have enough light to play by. Then there’s the issue of rain…