Most stories are about secrets being uncovered. They have an engine, something that drives the story that might be a secret all its own, but at the end of the day audiences are a bunch of nosy bastards who want to know all a character's secrets. Get enough characters together and a lot of hidden blood will be spilt.
Janet is having a party to celebrate her promotion. A few friends are coming over, including a lesbian couple who attempt at caregiving, an acerbic couple who like to talk a lot of shit, and a coked up husband with a violent agenda. As everyone waits for the husband's wife to show up, we get a spiral of secrets from affairs to diagnoses to pregnancies. Beginning with Janet shooting someone off camera, we know this whole night will end poorly.
Again, there's something about watching a room with a group of characters squirming against each other. Everyone is hiding something it seems, and the interplay between them all fills the space. The audience might as well be watching a tennis game as control of the environment is handed from character to character deftly and with charm. Throw in a reserved British attitude on the whole thing and have a pleasant evening.