ETIQUETTE 

I. When to Embellish 

It's obvious that if the Detective asks who the murderer is and s/he confesses immediately, this will be the shortest mystery party in history. At the same time, if everyone just makes it up as they go along, there will be no way to decode what really happened.  So use common sense.  Lie a bit to explain things that just don't add up, especially if you have been instructed to do so.  Fib to other characters who are nosing around where they don't belong.  Play innocent.  Keep secrets.  Just don't get carried away.  Believe me when I tell you that this story is complicated enough without embellishment. 
  

II. When to Cave 

Unless you are instructed otherwise (in writing), the time to cave in and admit what you know is when the Inspector reveals your secrets directly to your face.  His evidence should always be good, so there's not much point in keeping up the lie anyway.  Issue a few weak denials and then admit it all with a flourish.  This is your big moment.  Play your character to the hilt. 
  

III. Who to Believe 

Who can tell? Evidence never lies, but people do. Experts usually can be trusted to know their field of expertise. Everyone will try to learn things from you (maybe some things they already know) so doing a little investigating with third parties to confirm facts may help. The hotel staff can assist you, but they are not omniscient as each has had unique experiences before and at the Eaton and are just as likely to be as corruptible as the next person. Lastly, if you find yourself in a complete tizzy, consult the detective for help.

IV. Last Advice 

The best way to have a good time with this is to keep your character in mind.  Remember your motivations: are hiding something, trying to discover something, or both?  Circulate around and see what you can overhear.  Form alliances.  Get as much as you can while giving away as little as you can.  

Also, use your imagination! It will be impossible to convert everything to 1923 standards, so please expect a few things to look out of place (what's that stereo doing there?). If someone doesn't have a name tag, they are not part of the event so don't bother interrogating them (unless they deserve it).
  

Most importantly - have a terrific evening!