Practically how should one with Alzheimer's be cast for an Alzheimer's character?

They need to be able to remember lines and instructions. Which someone with moderate to advanced Alzheimer's cannot do. Even mild Alzheimer's - my granddad has that. And he can't remember conversations from a few minutes ago. So how could he be expected to act in a major motion picture let alone a role like Still Alice where the character has to show the progression of the disease from being completely cognitively healthy and in fact extreme intelligence and social confidence (she's a Harvard professor at the start who does major public speeches in front of hundreds) to being very impaired cognitively and socially at the end.

I think there are some types of disability where it's just impossible to cast someone with the real condition especially if it's symptomatic.

Another example is active schizophrenia - sure it's an argument to be made that you could cast someone who is not in active psychosis but still has experience with psychosis, and if you can do that, great I think it should be encouraged. But someone in active schizophrenia could not be cast in "A Beautiful Mind" despite the movie being about schizophrenia.

And I say this as someone whose husband has dealt with psychosis and delusions and was considered schizophrenic by some psychologists. He could be cast in a movie now that he's undergone intensive IFS therapy. But many people with schizophrenia don't get to the stage where psychosis no longer negatively affects their life. They're either delusional or so medicated that they are lethargic and unable to think clearly.