Everyone is John

There's only one character to play: John, who is an ordinary office worker suffering from multiple personality disorder.
Each player plays one of the personalities, with their own stats and predefined (conflicting) goals. They are free to play "characters" from any setting or era.
In situations that are stressful to John, a competing dice roll determines which player takes over.
The other players can still talk and comment, and John will hear them argue inside his head.