For this game, each poster writes a very short letter from one character to another. Each poster writes a letter to a character as the recipient of the last letter, thus picking the next letter recipient.
Rules:
1. Be creative but short.
2. General forum rules apply.
Examples:
Dear Caspian,
Please remember to eat all your vegetables, and dress up warmly when it's cold. Don't shirk your baths or your lessons; it isn't befitting a prince.
Remember Aslan!
With love,
your Old Nurse
Next poster:
Dear King Edmund,
We found your torch; you left it on the chair under your Narnian clothes. Do you mind if we use it until you return to fetch it? We will, of course, guard it most diligently as befits our kingly honour.
Sincerely,
Caspian X,
King of Narnia
P.S. Is it waterproof?
Many thanks to Meltintalle, Pattertwig's Pal, and Ryadian for the game idea!

Let's start off with the last letter in the example. (The next person will write a letter from Edmund.)