I was surprised to find that how I've interpreted and run the game differs from videos I've seen of others' games. Not to suggest there's a right way or a wrong way, but this seems to strongly impact the flavor of gameplay.
The way I've run it is guided by this paragraph:
> Everyone is John is a humorous, competitive roleplaying game about playing the various personalities of John, an insane man from Minneapolis. One participant is the GM, or, in Everyone is John lingo, “Everyone Else.” All of the other players are Voices in John’s head.
Hence, the Voices are John, directly making choices and taking actions. The GM, despite the name of the game, is not John. The GM handles all other characters, describes the world, calls for rolls, adjudicates, and does the usual GM stuff.
However, in videos of games I've seen online, players will act only as literal Voices in John's head, while the GM plays the voice of "sane" John, whom the other players get to boss around.
IMHO, I'm running it correctly, and the results speak for themselves, with tales of misadventures told and retold years later. The other version, to me, is more one-note and less worthy of replay. To each their own, of course.