-If you are a suspect, you will be asked to dress in a particular fashion.  You might want to add props as well. 
(for example:  a camera, if you are “the photographer”)

-If you are not a suspect, you will be invited to dress as a detective, police officer, reporter (from any era, fiction/tv/movie)  *or in theme.  (*Example:  if it is a 50’s theme you could dress in any outfit pertaining to that time frame.) 

Bring along your invitation and all the information it contains, as you will need to refer to it.