I first saw this movie in the theatre when it was released. Back then, as stated by another reviewer, the concept of human cloning was way out there. Seen today it certainly doesn't seem so outrageous.
Peck lives up to his reputation with this white-faced, black-haired, insane profile of Dr. Mengele. Mengele in person wasn't nearly as impressive! It is a refreshing change from Peck's usual roles, about as far from Atticus in "Mockingbird" as you can get! His cold and calculating confidence in himself and his work is still a standout among movie thrillers. Back when this film was made, the rumors of an international Nazi cabal operating out of S. America would still made the papers. Now, more than 50 years since V-E Day, it's a little less credible.
Olivier is great, as always. He plays to character, at least for an American stereotypical Jew, though possibly not for a European. Lilli Palmer is beautiful; no amount of effort to make her look shabby would succeed. I particularly enjoyed Uta Hagen as the imprisoned Nazi operative; I wished there was more exposure for her.
There wasn't a weak part in the cast of the film; everyone plays to a high standard. It was a pleasure to watch when it first came out, and doesn't appear too dated now. And, in some ways, it is even more believable now, particularly re cloning!