• the monster is a shapeshifter
• it has stack (as in the data structure) of targets
• it's always walking straight towards the target at the top of the stack (peek())
• the target can have sex with someone else to make them the new target (push())
• if the target at the top of the stack dies, the previous target is the target again (pop())

Beyond that, the writing and cinematography just let the audience play along. The characters are deliberating their plans on how they would deal with the monster, letting you also think about what you would do in their situation. And the camera likes to slowly pan around the people talking so that all the while, the audience is scanning the background looking for the monster. It can look like anyone, and they constantly, and deliberately put extras in the background walking directly toward the camera just to make you go "oh shit! Is that it right there? Hey, pay attention, we need to move!"

It's just such a fun, unique experience. I don't know of another horror movie experience quite like it.