Interesting to rewatch this immediately after reading the novel on which it's based (currently in print in an English translation under the title "Vertigo," by Pierre Boileau and Pierre Ayraud, who also wrote the source novel for "Diaboliques"). I had read that the film was only vaguely inspired by the novel. That's not true, really--most of the story beats in the film are also in the book. The endings are different, though, and the film's is more emotionally devastating.
I first saw this as a teenager in the late '70s on a reissue, in a theater with a very large screen (though I can't honestly remember if it was 35MM or 70MM). I had already read the book (and even the "Making of 2001" book), and I loved it. Can't say I always "got" it, but I knew it was an overwhelming work of art.
I've seen it on video numerous times since then, letterboxed but, c'mon, you need…
I wish I could have been with an audience on this movie's premiere just to see the reaction when the first head exploded. I can only imagine the bedlam that must have erupted.
Strong script, tautly directed. Patrick McGoohan is a joy to watch, as usual (I'm a huge fan of "The Prisoner"). Stephen Lack is a stiff, but sometimes he makes the lack of affect work for him. Other times, he just looks extremely constipated. Painfully, perhaps lethally constipated, especially while he's scanning.