Ruined by the second half
I enjoyed the first half very much. A failed playwright developing an idea to kill a young talented writer, the young writer coming over, the wife trying to talk her husband out of it, the young writer slowly realizing what's going on - that was great stuff. But then it all ended abruptly. When Caine started to strangle Reeve I was thinking: what, he's killing him already, but it's only half way through the movie! I think they should have stretched the Caine-Reeve-Cannon interaction for longer. The strangling of Reeve should have happened later, and the movie should have ended with the revelation that Caine and Reeve actually planned to murder the wife. The infamous kiss between Caine and Reeve could have been the very last shot. All what came after the wife was buried, was completely unnecessary in my opinion. And that old German woman - why did she even had to be in this movie?
I realize, this criticism actually applies more to the play, than the movie. But the makers of the movie should have understood that the play drags on for too long.