Oh, so that was Montgomery Clift.
It's always comforting to finish watching a Hitchcock film, and come back to his interviews to find that you feel the same way about his film that he did. While I don't think that it should never have been made* (due to disbelief that the priest would not speak up; a view that was apparently held by everyone except the Catholics), I do think that it was heavy-handed and lacking in humor. That tone, that humor and sense of irony, is one of the strongest defining characteristic of his films, and something that he touches on over and over throughout his talks with Truffaut. Its absence (for the most part) is definitely felt in I Confess.
But so yea, Montgomery Clift. phew! Not quite Marlon Brando-level magnetism, but close. His intensity and dignity really carries the role. I didn't mind Anne Baxter, although Hitch did. And Karl Malden, <3. The film also looks beautiful. Aside from some heavy-handed imagery and a surprising overuse of canted shots, the cinematography is really beautiful, and the settings look great - it was shot in several churches in Quebec.
Also interesting - some things that I had suspected were then confirmed in the wiki article, so if you have seen the film, I recommend taking a look (first paragraph here).
*FT: Then would you say that the basic concept of the film was wrong?
AH: That's right; we shouldn't have made the picture.
AH: As a matter of fact, it was difficult [trying to reconcile the criminal and religious elements in the screenplay], and the final result was rather heavy-handed. The whole treatment was lacking in humor and subtlety. I don't mean that the film itself should have been humorous, but my own approach should have been more ironic, as in Psycho - a serious story told with tongue in cheek.