The always aloof Alan Ladd, a former laborer, preferred the friendship of film crews rather than other actors or studio execs. Yet he was able to form lasting friendships with a few of his co-stars, especially William Bendix who accidentally cold-cocked Ladd during a particularly vicious fight scene in this film. Ladd was so taken aback by the sincerity of Bendix's apologies that they formed an immediate and unlikely friendship. They even purchased homes across the street from one another at one point. According to Bendix's wife Tess, the bond was strained in later years after Ladd's wife and manager, Sue Carol, made an offhand remark about Bendix's lack of military service. Stuck in the middle, it would be a decade before the wounds healed between the two. By then, Ladd was career down and self-destructive, leaning heavily on Bendix, who was thriving out of town frequently in the 1960s with stage work. Bendix's heartbreak was evident in the wake of Ladd's premature death (and probable suicide) in January of 1964. Bendix's health failed quickly and he too died (of bronchial pneumonia) a week or so before Christmas that same year.
In one of the scenes where William Bendix is punching Alan Ladd's character, he actually connected and knocked Ladd unconscious.
The title of this book and movie is most obscure; thankfully, its meaning is described by Richard Layman in his book, Shadow Man: The life of Dashiell Hammett. A glass key symbolizes an act or experience which cannot be reversed or forgotten. It is a key made of glass which allows one entry to a room or a building, then shatters after use. Skeleton keys were used for many years before and after this story to lock doors from both sides. Hence, a skeleton key made of glass which breaks in a lock will prohibit the locking of a door and prevent one from leaving the room. Therefore, once in the chamber, one is compelled to see one's choice through.
Many of the set elements used in the Clyde Matthews' summer house were from the "Holiday Inn" house, on which production closed about three weeks earlier. They were used again weeks later in I Married a Witch (1942), which also was produced by Paramount but released through United Artists.