After mulling it over in my head for a day now, I think I am in agreement with you, Pendragon. If I was going to DM something like this I wouldn't go as far as to remind or warn players about the guest player character, but I would probably allow them some clues provided they inquired about the new character. These clues might be Spot checks or Intelligence checks or other checks giving the characters a chance to be a little wary. Even if the guest player's character is caught doing something like trying to steal or that he has magic on his person, I think our players would have been much more cautious, but still would have no idea what the guest player was up to or what type of being he was. This would still allow the guest player some element of surprise over the player characters. So he could still use surprise to his advantage, but the other players would have had a chance to suspect something. I guess thats just the way I would do it if I was DMing.
