I suspect that the majority of the people interested in the film will be those who know the show already. That's not meant as a criticism, just an educated guess. I don't really assume this film will have substantially wide appeal outside of diehard (and mostly younger) musical theater fans. I'd love to be proven wrong, but...
(I'm not quite comparing DEH to Hello Again, but I think there were about 8-10 of us in the theatre the day I saw that movie. I had done the show before. I don't know about the others lol. Not that I expected a packed house, but that was pretty brutal...)
