This is a clever directing job to make a film noir mystery into a good film.
Film noir often stumbled from being too Hollywood, with heroes and heroines automatically acting creepy just for the sake of acting creepy.
Here, we get the opposite. The best mysteries have the woman being the "woman of mystery", because that is part of being a woman, while being a man means having your mystery come out of your very straight forward approach.
Sterling Hayden often acted gruff for the sake of acting gruff. A modern day guy who thought there was always a camera on him.
But here he acts more like a believable man from the fifties, or any era before the twenty first century.
He becomes the "everyman" who is introduced to a mysterious world, which involves espionage.
The CIA and FBI are put in a likable light, which doesn't go over well with those who like the post 1965 cliché. Being 1957, this goes under the old cliché, which really wasn't around long enough to be a cliché. That's why such CIA and FBI characters are still fresh, and in the long run, more believable than the silly assassins of today.
What helps here is a great atmosphere. We get some road, and not too much of the cars. We have a few pit stops, and changing scenery, which makes this flow very well.