Sound (like music and background sound) and detail. The music and background sound is what sets the mood for the game. In horror games, the sound is sometimes uneasy to make the player feel anxious, and when something chases you the music turns into fast-pace and making the player scared and feel that adrenaline. The detail also is a great factor. The more realistic a game is, the more it feels like real life and determines if the player will get a real reaction from the game.