You and your roommates have discovered that someone in this house is secretly THE ALIEN.
Before the others arrive, cut pieces of paper equal to twice the number of players. Mark half and fold them all, then discard half at random.
Once everyone's there, name your characters. Make sure these are perfectly ordinary human names that will easily blend in with your fellow humans' names. Because you're human.
Then, each player takes one paper and looks at it. If it's marked, they are secretly THE ALIEN.
This may be more than one person, or even everyone. Being THE ALIEN does not mean you get to know who else is THE ALIEN.
This may be no one. Oh well! Guess you all suspected each other for nothing.
If this is your first time playing, pretend exactly one paper is marked.
Once you're all named and THE ALIEN knows who they are, it's time to call a house meeting. Start by explaining how you know THE ALIEN is among you.
The human players' goal is to uncover THE ALIEN.
THE ALIEN's goal is to cast off suspicion and not get kicked out.
Once everyone's in agreement, settle who's staying on the rent.
[please do archive! i might try to expand this later if there's interest, too.]