There are usually four positions in the game of Mafia, though they are usually filled by more than four people for larger games. Anyone in the game who is not one of these four is a townsperson.
The Moderator or God has all the power in the game, and is the only one who knows everything that's going on. He decides who the Mafia, detective, and doctor are (unless positions are chosen by cards). He also tells everyone when to wake up and confirms who is dying, being accused, and saved. When someone is killed off, the Moderator or God gets to have a little fun and make up how the person died, often described gruesomely and in great detail.
The Mafia is often made up of three people. When they awake, they all agree (by pointing silently) on who their next victim is. During the discussion period, they are supposed to accuse others of being Mafia so their own are not killed off. However, sometimes a Mafia member will accuse another member, or support a townsperson's accusation. If the game reaches a point where only Mafia members are still alive, the Mafia wins.
The detective is a bit of a tricky position. When the detective awakes, he guesses who is part of the Mafia (again, pointing silently), and the Moderator lets him know if he is right, often by nodding or shaking his head. During the discussion, the detective tries to persuade the townspeople to kill off anyone who he knows is Mafia. Generally, the detective does not want to reveal himself, because if he does, it is almost certain death for him in the next round. However, the detective is allowed to reveal himself at any point during the game.
The doctor is also referred to as the nurse or the angel. When the doctor wakes up, he silently points at the person he chooses to save. If he has chosen the person that the Mafia earlier chose to kill, the person does not die. However, if he chooses someone else, the Mafia-chosen person does die. The person the doctor chose is not affected. A selfish doctor will often choose to save himself. However, in some games , the doctor will only be able to save themselves a certain number of times throughout the entire game. But this is not a very popular rule.
Playing the Game
The Moderator or God is first chosen; often, it is simply whoever started the game. Everyone except the Moderator or God then sits in a circle. The Mafia, detective, and doctor are then chosen, usually in one of two ways: by taps from the Moderator while everyone else has their eyes closed (one tap is a townsperson, two is a doctor, etc.), or by the dealing of cards to each player (queen is doctor, king is detective, aces are Mafia, all others are townspeople). The second method is more often used because it is more fair, but the first method is used if there are no cards available.
The Moderator or God then tells everyone to go to sleep (close their eyes), and has the Mafia wake up (open their eyes). The Mafia then choose someone to kill. They go to sleep, and the detective wakes up. He chooses someone to accuse, and then goes back to sleep. The doctor awakes, chooses someone to save, and goes to sleep. The Moderator or God watches all of this, and confirms (or doesn't) the detective's guess. No one else playing can see this. If a player peeks, he is said to be killed by aliens and is out of the game.
The Moderator or God then has everyone wake up, and announces the death of whomever the Mafia chose (unless said person was saved). Then comes the discussion period. Everyone has a chance to throw accusations before voting on who they think is a member of the Mafia. The person with the most votes lynched by the townspeople. Depending on the Moderator or God, he reveals what the position of the person killed off was. Everyone then goes to sleep again, and the process is repeated.
After a person dies, they are not allowed to say anything or make any signs towards who the Mafia is. If they do, they are said to be desecrated and either the moderator or the townspeople choose an embarassing thing for s/he to do once the game is over. They are allowed to watch the waking up and sleeping of the different people. The game ends either when all the Mafia members are killed off, or when there are only Mafia members still alive.
The most common variation is the amount of Mafia members there are. This depends on the amount of people playing. There should usually be at least three townspeople, and at least one more townsperson than there are Mafia members. The usual number of Mafia is three. If there is a particularly small group, there may be no doctor. In a very large group, there is occasionally more than one detective or doctor. Like the Mafia, however, they must agree on who they are choosing.
The Wikipedia article on Mafia states that there are more than 4 basic roles. There rarely are games which have these roles in CTY.
There are also other games that go by this name that involve handshaking and winking. They are rarely (if ever) played at CTY, and are not as much fun.
At LOS.08.1, the Game Theory B teacher Randy Hall modified the original rules of Mafia and added other roles. The ones that survived the entire session were Mafia Recruiter (self-explanatory), Mayor (who had two votes instead of one), Dinosaur (who had two lives), Dam Keeper (who killed surrounding players when he/she died), Mafia Dam Keeper (same as Dam Keeper, but part of the Mafia), and Ghosts (who were dead players). Ghosts had the ability to haunt players, the newest Ghost (most recently dead player) having more power than the others (in case of a tie). When a player was haunted, she/he would be saved from the Mafia if he/she were selected to be killed, but he/she would also be unable to speak during the next round.
Since 13.2 at LOS, some students have been playing with a position called the Potato. During each round of the game, the Potato is given the option of commiting suicide and taking any person they choose with them. The origin/reasoning behind this position is unknown.