This is my all-time favorite classic movie. It has an very sophisticatedly entertaining plot line, the casting is superb, the pace is breathtaking, and it deals with a subject (euthanasia) that is still controversial today. The story is a fine example of "black comedy", where a socially unacceptable idea is shown in a very entertaining manner.
The story is set up brilliantly right from the get-go; where a 'certifiable' publicly-acclaimed bachelor is secretly getting married. The personality of the cast is excellent. I know that Cary Grant reckoned this was his worst movie, saying it was more of a "Jimmy Stewart-type part"; but his spot-on comic timing and professional style hamming plays the role to perfection. Also co-starring in the movie is a brilliant Peter Lorre as a maniac doctor and Raymond Massey as the psychotic brother. Most critics have attacked this film by saying the script refers to the psycho being a Boris Karloff look-alike, highlighting the fact that Boris played the role is the original stage play. However Massey plays the role to deadpan perfection, and the humor of the scenario still works.
My favorite scene is the self-referential one where Mortimer (a theater critic)is describing "bad plays (and movies)". If you watch the background action, and pay attention to the dialog, the ironic situation is brilliantly realized. This film also has my personal favorite quote, said by Cary Grant as Peter Lorre frantically tries to warn him of impending doom; "Stop underplaying - I can't hear you!"