That's a hell of a double feature, DR MichaelG. Glad you enjoyed Marnie. Like DR John G. says, the opera is based on the original novel, but the film gives you plenty to go on and I believe you'll enjoy the opera just fine. I was one who always liked the film, and I've developed even greater respect for it over the years.
The Trouble With Harry was a tougher nut to crack when I first saw it as a kid. Going in cold, I was wrongly expecting it to be something with more suspense or perhaps even a little bit scary. I think about ten years passed before I was seeing it at Hitchcock festivals in L.A., and of course I "got it" and liked it just fine then.
