Susan Strasberg as Caulder's love interest is sympathetic, and very pretty, but isn't given much screen time outside of being a nanny for Caulder's daughter.a minor complaint to be sure. Henry Fonda's exchanges with Caulder are similar in their edge and that makes for an understanding of what Harry is up against in trying to stop the bomber. That credit should go to the screenwriters.
While the Widmark character of Hoyt is a pretty much by the book portrayal of a federal dick, his sardonic exchanges with Segal lend an heir of authentic, yet begrudging mutual respect. The Caulder character is identifiable because of his family and work failings while Bottom's soft-spoken psychopath (which probably would be portrayed as over the top if the movie were to be made now) is much more in tune with an understated realism that most contemporary madmen you see on screen today lack. It works because none of the characters are grossly exaggerated caricatures but everyday men with foibles like struggling to quit smoking. I'm sure part of that is the nostalgia factor as it seems to capture a time and place(s) pretty well, but there's a bit more to its appeal to me than that. This is probably my favorite childhood film having seen it 30 times at the theater during the summer of '77 (to put it in perspective I only saw Star Wars 12 times) and it still holds up very well.