You can make a case that Talman developed feelings for Susy and that this is part of the reason why he lets her go. Especially in stage productions, where their relationship can be played countless ways by the actors.
This trope also applies to Roat and Lisa's relationship in the movie. Compared to the play where Roat and Lisa never knew one another before he was assigned to kill her, the 1967 film establishes that the two were partners-in-crime and hints that they might have been more than that. Little details in Arkin's performance, such as his detached line readings during the moment where the men go to dispose the body (as opposed to his gleeful sadism in just about everything else he does) or his reluctance to discuss Lisa when Susy brings her up during the climax (as opposed to in the play where he gloats about murdering her), imply they might have been lovers and Roat is just a teensy bit regretful that he had to kill her. Not that this makes him any more sympathetic, of course...
Complete Monster: Harry Roat is an ice-cold sociopath out to find a doll filled with heroin, which has ended up in the possession of a blind woman named Susy Hendrix. Roat romances and kills a woman, then brings her two exes in and sets them up so they'll be forced to work for him. He then makes Susy think that her husband's an adulterer and uses his partners to psychologically torture her and make her afraid to leave her own home. Roat happily murders his way through anyone in the path, and when his unwilling partners attempt to finally be rid of him, Roat disposes of them before they can lay a hand on him. Roat then spends the climax of the film psychologically torturing Susy before trying to murder her. Even at the end, Susy realizes the money is simply an excuse: Roat is a natural sadist who enjoys evil for the sake of evil.
When Mike discovers Lisa's dead body in the closet, her clothes are torn open, showing her bra beneath. Considering Roat's predatory behavior towards Susylater in the film, it seems very likely his murder of Lisa may have been preceded with some form of sexual violence.
The pink scarf Roat uses to torment Susy with seems like a weird thing for him to be just carrying around— until you realize Lisa was strangled and that he probably used the scarf to get the job done. And right before marching Susy to the bedroom, he leaves his knife on the table but still has the scarf in his pocket, suggesting he was likely going to use it on her as well.
Nightmare Fuel: The last 15 minutes of the movie, as Susy struggles to kill every light source in her apartment before Roat can kill her. The final minute of battle between them is in total darkness.Stephen King named the Jump Scare in which Roat jumps out from the shadows at Susy as the scariest thing he's ever seen on film.
Even if you're not blind, this movie is terrifying. Especially the early scene where Susy gets home, unaware that there are three men hiding in her house, and a dead body hanging in her closet.
During that same scene, there's a bit where Susy grabs a scarf from the closet without realizing that the aforementioned dead body is RIGHT NEXT TO HER. Then she puts the scarf on and it brushes the corpse's face! ACK!
Retroactive Recognition: During the movie's opening credits, as Lisa is running after Sam in the Kennedy Airport concourse, a little kid in glasses can be seen in the background tossing a football to himself. That kid is an 11-year-old Robby Benson.
Unintentional Period Piece: Most modern productions of the play explicitly set it in the 1960s (the decade the play was originally produced), since tropes like Cut Phone Lines and the need for the villains to rely on a phone booth rather than cell phones do not apply in the twenty-first century.
Unintentionally Unsympathetic: Susy's husband comes off as insensitive (even his actor thought so), especially during the final scene where he demands Susy walk over to him even though the apartment smells like gasoline, there are two corpses of strange men littering the room, and Susy is in shock and covered in blood behind the fridge. And Gloria's tantrum isn't justified by what we see (at least, in the film, where the animosity between them is softened a bit; in the play, Susy's behavior towards Gloria is much harsher before the girl finally snaps).
by Gustav Mahler. It remains unclear if Henry Mancini intentionally meant this as a Shout-Out or if it was a just a coincidence.