Do you know whats so wierd? I have a friend who is starting up a game similar/ exactly the same to what your doing. What he is doing as well is have us, the players, make the characters and their backstories and then he will distribute the characters to other players randomly. Anyways, my advice to you is to actually make sure there are repercussions to their actions if they do something stupid/ out of character. For example
-lose class abilities
-go to jail (in game)
-give less XP to the people who don't try to roleplay.