If you want Alan Rickman's autograph, it's probably not best to mention Harry Potter at all in your letter. Send a blank index card that you can photo with a photo of Snape, if you want him for your collection. He rarely replies, but people have gotten replies before. So good luck!

Well when I wrote them Dan sent a preprint and Rupert didn't reply at all it's been 4 years and I'm still waiting

I would like to know that too, thank you!
I have an Emma Watson preprint, and a robbie coltrane preprint already

Oops, sorry for being unclear i would like to know which ones reply authentically please
thanks very much

Maybe it would help us if you specified what kind of tips you need ;) I mean, do you need help with the general request-sending process, or do you need tips like which cast members reply authentically, which don't reply at all, what addresses you should write to, if there's anything specific you have to do when sending to them and this kind of stuff?

Subject says it all