I agree with Bulwark, however if you are concerned about soemthing being overheard you can use the private mail system to each other.
HOWEVER!!! you MUST send a copy of any private massages that you may have. In the even that someone is actively attempting to overhear your conversation, I need to know what was said so that I can pass it on.
With that said, if you are attempting to eavesdrop on someone you MUST make me aware of it so that I can convey what you hear, either in something that is posted publicly or privately.
I would PREFER that you trust your fellow players and keep all conversations public so that I dont have to keep track of anymore posts than needed.
But exceptions can be made.
If for example a couple of characters are going to be doing an extensive side trip away from the party, we can even go so far as to run a seperate thread for that trip. Then you can rp together to your hearts content in the IC rather than in private posts.