Mine usually have some very basic ground rules. Players are free to introduce new elements to the game, as long as they do not require any other player to make changes to their personality or history. If I'm an assassin waiting for my mark, you're free to wander in and ask me for directions to the nearest gas station and ruin the whole thing for me as my mark walks by and gets away during this exchange, but you're not free to decide that I was there to meet you, because that changes my own character's history to include knowing you and having made plans with you. The only rights I tend to explicitly deny to other players are passing significant amounts of time, and creating global events that are going to affect all other characters, IE, no you can't make a dragon attack the ****ing bar while I'm waiting for my mark.

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To relate it to a video game, say Elder Scrolls... If you didn't have to hit the ~ key to open the console to make this post happen, it's fair game.