If dream sharing is impossible, why not make those that claim it is possible.
Ask their friends what they dream about, BEFORE going like.. "Did you dream about this?"
if that IS the case, then voila.

Now before you read the rest i'll quickly answer your question:
Yes dream characters can lie, if your mind makes them lie, and no it never really happened to me, because I do not talk about personal stuff a lot in dreams, but I did lie to a dream character once in an un-controlled dream.

OR an other possibility would be to ask your ''friend'' in your dream, something you do not know.

Eg.: There's a girl from your class and you share a dream with her.
Ask her who she likes, or who her very best friend is, or something like that.

Simply confirm the information she gave you, and voila.
(Just make sure she hasn't told you before)

Now I ask YOU, why would dream sharing be so impossible

Originally posted by Theotherguy
Dreams happen inside your own mind and are experienced only by your own mind. There is no mysterious "force" of dreams or a "place" that you go to, its simply imagery created by your own mind for you own purposes. Dreamsharing can be easily disproved by the afformentioned method of writing down and remembering a password, providing it to the supposed dreamshared person in your dream, and then seeing if the person in the real world remembers it.
Remind me: Didn't people a long time ago, think that the world was flat? They were CERTAIN of it?
You might want to ask what relevance this has (Since the world & dreams, what the hell?)

But I just want to show you the point: As long as you do not have the ''skill OR technology'' to find out whether its true or not, everything is possible.
You do not know if they only exist in your mind, you only know that little lines appear on a scan device when someone sleeps.