Here's the basic idea:

In a frontier town we are all in, I am the town's lazy Sheriff. I won't arrest someone unless I have good evidence (or reliable eye witness accounts) of a crime that happened or will happen, or if I witness it. At least one person will be a criminal. They will attempt to hide their crime, and the town will have to discover the perpetrator and their crime.

In order to insure that there is no metagaming, if you want to be a criminal, PM me with what crime you want to commit and why your character is doing it after you have a character posted, though not necessarily approved. If I like the idea, I will approve it. You will then, in your IC posts, go about your life, though giving your character the opportunity to prepare by having them do things which give them the opportunity (visit the bank prior to a robbery, go into an alley where you hid a gun in case you are caught, etc.) You will then PM me the details of what you did and where, and I will feed them to any character that discovers them (ex: A guy walks down the same alley and I tell him that he sees where someone had dug.) Be sure to give detailed descriptions of what you do in town, especially when it intersects with criminal activity or prep activity.

The game will be over when all of the criminals are either caught or have escaped. For the first round, I'll accept two crimes so that it isn't too complex.