It was originally a play, and it’s not unreasonable for there to be different adaptations. When I was a teen-ager, I saw Bob Newhart do HARVEY (as a play) and he was fantastic.
The Harry Anderson remake was dismal, in part because of poor direction – Harry was always in center of frame, which made Harvey clearly illusionary. The Jimmy Steward version has two-shots, lots of them, treating the invisible Harvey as a real character. I think Jimmy Stewart nailed it, and will always be Elwood, but I have no problem with other interpretations… so long as they’re true to the original and done well.
The “Harvey is real” argument can only be made from the movie, not from the play. The play has (IIRC) a delicious ambiguity.
Now there is an interesting take: yes, now you remember, you have seen Harvey before, and more than once. But something about him makes you wonder whether he really was a pure figment, or maybe the acid actually did open your mind to another plane of existence. Or maybe both are true - Harvey is in a way part of you, but has an independent identity as well, something like a projected schizophrenic sub-personality or conscience, as if your liver developed a personality of its own.
Ray Bradbury did an interesting reversal of this in his Martian Chronicles where Martian hallucinations are visible to everybody, so when the Earthmen arrive and show their spacecraft they are still locked away with the rest because their hallucination is just the worst to get rid of that anybody has ever seen.
