Alfred Hitchcock was famous, or perhaps notorious, for making changes to novels and short stories when adapting them into films. Vertigo (1958) is a classic case in point. It was based on the 1954 novel D'Entre les Morts (From among the dead) by Pierre Boileau and Thomas Narcejac. The novel is centered on a detective, Flavières, who is hired to follow Madeline, the wife of an old friend. Flavières soon falls in love with Madeline, who believes herself possessed by an ancestor who committed suicide. When Madeline herself dies, Flavières is bereft; when he meets a woman who reminds him of Madeline, he begins to obsessively make her over in Madeline's image in a doomed attempt to recapture the past.
There are many differences between the novel and the film. Hitchcock altered the novel's settings and character names, and added characters and scenes that didn't exist in the book. Perhaps the most radical change was in the timing of the revelation of Madeline's identity: in the novel, there is a surprise twist at the end, while in Hitchcock's film, that twist is revealed at the beginning of the film's second part. As Hitchcock told François Truffaut, "Everyone around me was against this change; they all felt that the revelation should be saved for the end of the picture…We're back to our usual alternatives: Do we want suspense or surprise?"  As ever, he had no hesitation in making substantial changes to the source in order to make it more Hitchcockian.
One of those changes relates to a key scene in both the novel and the film: Madeline's attempted suicide by drowning. In the novel, Madeline is standing alongside the Seine when she suddenly steps off the quay and plunges into the water. Flavières, who has been following her, leaps in after her and pulls her out. She is conscious:
Her eyes were open, gazing pensively at the sky, as though trying to recognize something.Flavières carries her into a nearby café. There Madeline is able to stand, shakily; the proprietress gives each of them some dry clothes to change into, and they return to Flavières's flat. As they enter, the phone is ringing: it's Madeline's husband, who is concerned about her lengthy absence.
"You're not dead," Flavières said simply.
Her eyes turned towards him, and her thoughts seemed to come back from some other world.
"I don't know," she said softly. "It doesn't hurt to die." 
In the film this incident is altered and elaborated in ways that are highly significant. Madeline (Kim Novak), followed by detective Scotty Ferguson (James Stewart), drives out to Fort Point underneath the Golden Gate Bridge.
Suddenly, she leaps into San Francisco Bay:
Scotty jumps in after her and pulls her out:
He carries her unconscious body to her car, where he tries to revive her without success:
There is a cut. When we next see Scotty, he's in his apartment; Madeline is in his bed, and her clothes—all of them—are drying on a line.
Madeline is awakened by the ringing of a phone: it's her husband, calling Scotty to find out where she is. She is shocked, first to discover that she's in a strange bed:
And next, by her realization that she's been completely undressed:
This moment—awkward and embarrassing for both of them—is also the moment when love begins to dawn between Madeline and Scotty.
I had thought that this scene was simply one more example of Hitchcock improving on his literary sources: it provides a basis for Madeline and Scotty's subsequent intimacy, while at the same time providing hints of Scotty's obsessiveness and perversity (particularly with regard to his minute attention to women's clothing).
But I think I've discovered that this scene has a cinematic as well as a literary source. Hitchcock seems to have adapted pivotal elements of this scene from another film: Hot Saturday (1932), a pre-Code film directed by William Seiter and starring Nancy Carroll, Randolph Scott, and Cary Grant.
In Hot Saturday, Ruth Brock (Carroll) tries to escape rumors that she's spent the night with rich ladies' man Romer Sheffield (Grant) by fleeing to the campsite of her earnest childhood friend Bill Fadden (Scott). Bill has returned to town after seven years away to do some geological exploration (but he's too shy to declare his real reason: his love for Ruth). A violent storm is raging, and as Ruth tries to reach the cave where Bill is staying, she slips and falls unconscious:
Bill hears a noise, investigates, and carries the unconscious Ruth into the cave:
Bill tries to revive her, without success.
When Ruth awakes, her clothes—all of them—are drying on a line:
Ruth is at first shocked to discover that she's in Bill's bed:
And next, by her realization that she's been completely undressed:
The parallels between these two scenes seem too close to be coincidence. Although I know of no incontrovertible evidence that Hitchcock was familiar with Hot Saturday—it does not appear in the indexes of any of my small collection of books on Hitchcock—it was one of Cary Grant's first starring roles. Grant went on to star in four of Hitchcock's movies, and occupied a special place among Hitchock's leading men. Donald Spoto, in his biography of Hitchcock, wrote that "Cary Grant...represents what Hitchcock would like to have been."  Hitchcock himself, in an after-dinner speech given at the Screen Producers Guild in 1965, jokingly referred to Grant as his alter ego: "You may be sure that in securing an actor for my next picture I was more careful. I gave casting an accurate and detailed description of my true self. Casting did an expert job. The result: Cary Grant in Notorious."  It seems inconceivable that Hitchcock would not have seen all of Cary Grant's early films.
We know, too, that Hitchcock—shall we say, obsessively?—restaged and reworked similar scenes over multiple films. Such recapitulations have been the subject of at least one book, the not-very-enlightening Hitchock's Motifs by Michael Walker. Perhaps the most famous example is the heroine dangling from the face of Mt. Rushmore in North By Northwest (1959), prefigured more than two decades previously in Young and Innocent (1937):
Eva Marie Saint and Cary Grant in North by Northwest:
Nova Pilbeam and Derrick De Marney in Young and Innocent:
In North by Northwest Hitchcock is restaging, on a grander scale, a powerful scene from one of his earlier films. What makes the Vertigo scene I've discussed in this post so unusual is that Hitchcock seems to have been adapting a scene from another filmmaker's work, rather than his own.
Boileau and Narcejac's Vertigo can be borrowed with a free account from the indispensable Open Library. Hot Saturday is available on DVD in Universal's Pre-Code Hollywood Collection, and is worth seeing as an early example of Cary Grant's screen persona (as well as for its eyebrow-raising sexual mores).
1. François Truffaut, Hitchcock, Revised Edition. Simon and Schuster, 1985, pp. 243-244.
2. Pierre Boileau and Thomas Narcejac, Vertigo. Translated by Geoffrey Sainsbury. Dell, 1958, pp. 41-42.
3. Donald Spoto, The Dark Side of Genius: The Life of Alfred Hitchcock. Ballantine, 1984, p. 442.
4. Sidney Gottlieb, ed. Hitchcock on Hitchcock: Selected Writings and Interviews, University of California Press, 1995, p. 56.