Hi destiny43304
I think TheMailman has answered most of your questions, but I'll just clarify
destiny43304 wrote:1. If I want his autograph, should I send him his photo for him to sign? Or he will send me a signed photo himself?
Normally, celebrities will have their own photos to send out, so sending a photo or other item to sign is optional.
destiny43304 wrote:2. He will go to filming outside UK after New Year. But my letter shall arrive before Christmas day. Will he receive my letter before 2007? If not, he'll only receive it after say half a year. I'm afraid at that time, the letter will be lost. Also, the Christmas card will be meaningless. What should I do?
Send out your request as normal. It's the thought that counts, and William won't think less when he sees a Christmas card - whether it be this year or half way into next year

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destiny43304 wrote:3.
From receiving my letter to replying, around how long does it take? 'cause if it takes too long, it's still unlikely for me to get a reply since he's going to leave England in January 2007!

The time it takes for a celebrity to respond depends on how busy they are. You may receive a response within days, weeks, months or even years! Normally the waiting time is usually around 3 months.
Once you send out a request, all you have to do is wait as the situation is now out of your hands (I know this can be hard).
destiny43304 wrote:4. It is more troublesome if I include IRC instead of stamps. Will the actor not reply me due to this reason? Has anyone get a reply by sending fanmail overseas with IRC included?
It's better if you send a
SASE (stamped, self addressed envelope) as it increases your chance of receiving a reply (provided you can get UK stamps). If not, it's perfectly fine to send an IRC with a
SAE instead, and is better than sending no return postage at all.
I always try to include return postage with my requests, as it can increase your chance of receiving a reply as you pay for postage. IRC's are a little more troublesome than sending a SASE, but if the celebrity doesn't wish to use it he/she can send it back to you
Good luck!
nowandagain [rockon]