Alterate reality.  For example, cities don't have to match their real-world layouts (it's too much work to deal with a realistic city you've never been to), and some things are legal that aren't here, ex: some drugs and prostitution is legal.

I'll play the NPCs such as witnesses, victims, crime lab workers, the police chief, suspects, etc.  If you want to play one of those rules, though, I'll give you the evidence and let you "discover" it.