I. When to Embellish 

It’s obvious that if the Inspector 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 is always 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 

The servants are all corrupt, so they can bribe you, blackmail you, betray you to other people, tell other people’s secrets, and generally create havoc.  However, they aren’t allowed to lie, so if they tell you something, it’s true (or at least they think it is).  The Inspector never lies, though he’ll pretend to be deceived while he’s waiting for you to give yourself away.  Anyone else is risky. 
  

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.  
  

Have a terrific evening!