Assassins is a game that encourages paranoia, not least because the players rarely know more than a few others who are engaged in play at any one time. The basic premise is to 'kill'1 people with 'weapons' such as water pistols and toy swords.
The Process of a Game
Someone decides to organise a game.
An announcement of some sort is made in order to get players.
Everybody who wants to play reports to an organisers' desk and has their name taken. Often, they will have their picture taken as well.
The organiser prepares little slips of paper for each person, bearing their name and picture (if taken). Other information may be added at the organisers' discretion.
Each player is given a slip showing someone else, and they must then hunt down and kill that person. At the same time, they will have someone hunting them.
When an assassin kills their target, they take the victim's target and attempt to kill that person. The targets should be given out in such a way that nobody will ever end up having to kill themselves.
After a kill is made, the assassin and the victim both report it to the organisers. The assassin may add as many lurid embellishments as they wish.
After a certain amount of time has elapsed - usually a few days - the results are collated and the winner is generally the player who scores the most kills (though this may not necessarily be the case - see below). If there is only one surviving player, however, then they are declared the winner.
Making a Kill
When a player finds their target, they must kill them with some sort of implement. They do not actually cause harm, of course; they merely look like they do. For example, if the implement is a plastic knife, the assassin would stab or slice at their target. Any item can be used to make a kill: water pistols, apples (the stalk looks like the pin of a grenade), cardboard boxes with words like fridge or very heavy written on them, or even mobile phones (they give off radiation). Generally speaking, there are no restrictions on what implements may be used - just because one person uses a water pistol does not mean someone else cannot use a plastic knife.
It is well to be aware of any local laws regarding weapons. In the UK, for example, your totally harmless foam gun might get you in trouble with the police if it looks too realistic.
It is entirely within the rules for assassins to work together; for example, one could lure the target to a special place for the other to administer the kill. Kills only count, however, if they are administered by the proper assassin. To explain this, let's imagine a game involving three people named Alex, Brian and Carol. Alex is assigned to kill Brian, and enlists Carol's help to do so. In this case, only Alex can kill Brian and claim the point; anything Carol does is regarded as a distraction.
There are no 'zombies' in assassins; those already killed cannot do anything. Thus, if Carol had already been killed, she could not declare an alliance with Alex. Also, if she was to be killed after the plans against Brian had been made, she would not be allowed to carry them out.
Assassins is a fairly flexible game and, as an organiser, you can invent pretty much any rule you like. Here are a few to get you started:
Extra points may be awarded for killing someone in a particular way; for example, by using something green, or a piece of computer hardware. Extra points may or may not be awarded to a player each time they kill in a special scoring way. The winner is then whoever has the most points at the end, rather than simply whoever has the most kills.
In this variant, a few extra players are roped in as bounty hunters. They can track down any assassin and administer a kill. When this occurs, however, the assassin has one hour to find and kill their target, otherwise they die and must give their target to whoever is tracking them. Any assassin may ask a bounty-hunter to perform their task on any other player, although no player will get a point if their target dies by bounty hunter. Bounty-hunters help keep a game moving, and there may or may not be a separate prize for the most effective of them.
In this variant, described in the novel Big Trouble by humorist Dave Barry, the assassin must kill the victim with precisely one other person present as a witness. This means you cannot simply walk up to someone and shoot them if there are several people nearby, regardless of whether or not they can play the game. The witness need not be a player, but should know how to play.
Where and When to Play
Fantasy and sci-fi conventions are the perfect places for a game of Assassins, as there will be plenty of people in fairly close proximity and many of them will be the sort of person who likes a game such as assassins.
If you are in college, then a game of assassins could be a nice charity fundraiser during RAG week2 if you charge a small fee to enter. Given the degree to which students move around, especially during RAG week, kills may be rare.
Assassins can also potentially be played in a school. Heightened patrols of teachers and prefects can be used to ensure it doesn't get out of hand. On the other hand, if people are mature enough, the game could be more fun - and easier - if the administration does not know.
If your boss is lenient, you might see about organising a game where you work; it would certainly make the average week more interesting.