Each player is assigned a character.
There are five characters: George (Richard Burton), Martha (Elizabeth Taylor), The Biologist (George Segal), Honey (Sandy Dennis), and George & Marthas apocryphal child, whom well call The Little Bugger.
Whenever your character drinks in the film, so must you. (In the case of The Little Bugger, you must drink every time he is referred to in the dialogue.)
The assignment of characters should be random. e.g., a party might have three Marthas and no Honeys.
The best way to assign characters would be to either print out the cards (via the links above) and have people pick at random, or click this lovely little WAOVW? Character Generator that I just made for you because Im thoughtful like that.