Politely refuse.
WWYDI you saw a prince named Caspian on the news?
"How did I get into Narnia?"
WWYDI you managed to walk deeper into a closet than actually existed?
Keep it as close to the book as possible.
WWYDI you met a family whose last name was Pevensie?
"YESYESYESYESYESYESYESYESYESYESYESYES!!!!!!!!!!!!!!!!!"
WWYDI Miraz invited you to be his guest?
Decline, tuck tail, and run!
WWYDI a picture came to life?
Be a little freaked out and hope they were intelligent talking animals.
WWYDI you were given a magic horn for Christmas?
Treasure it forever!
WWYDI you dreamed of a huge lion picking you up in his mouth and showing you something out your window?
Mention it on the MLIN thread.
WWYDI a nasty cousin suddenly got un-nasty?
Have /so/much fun!
WWYDI you were invited to the Council at Dancing Lawn in PC?