In interviews, 'Alfred Hitchcock (I) said that an RKO executive ordered that all scenes in which Cary Grant appeared menacing be excised from the film. When the cutting was completed, the film ran only fifty-five minutes. The scenes were later restored, Hitchcock said, because he shot each piece of film so that there was only one way to edit them together properly.
Despite the indecision over its ending, the film was a tremendous success, and more importantly Alfred Hitchcock had enjoyed a measure of creative freedom which he knew that he would not get at Selznick International.
The dog in the movie is a Sealyham Terrier named "Johnnie" and Alfred Hitchcock's own dog. The main male character is also named "Johnnie" and this name was Hitch's original idea for the movie title. Alfred Hitchcock kept Sealyhams for many years. In his movie The Birds (1963), Hitch walks out of the pet store with Sealyhams on a lead.
David O. Selznick was dissatisfied with Miklós Rózsa's scoring for the ski sequence in "Spellbound" and replaced it with a cue written by Franz Waxman for the final scene (Fast car driving scene) in this film.
The trivia items below may give away important plot points.
Unlike the novel "Before the fact", the film focuses much more on the psychology of Lina. For example, the Anagram scene in the film isn't in the novel. Another example is where the atmosphere becomes very dark when Lina reaches the house after visiting the land Johnnie and Beaky decided for their corporation. When Lina finds out that Beaky is alive, the atmosphere becomes a bright and joyful atmosphere while Vienna Blood waltz is playing in the background. Unlike the book, the film also focuses on the inner conflict of Lina. For Example, the scene where Lina talks to her father's portrait - "He didn't go to Paris. He didn't go to Paris I tell you." Unlike the novel, the film's focus on the psychological side of Lina makes it more ambiguous about Johnnie being a murderer.
In an interview with Dick Cavett, Alfred Hitchcock explained how he managed to call attention to the glass of milk containing the fatal dose of poison. He wanted it stand out so he had a glass made that contained a small battery operated light so the glass would show up in the long down the hallway shot.
Alfred Hitchcock originally wanted Johnnie to be guilty, but the studio insisted that the public wouldn't accept Cary Grant as a murderer. Hitchcock's original ending had Johnnie killing Lina by poisoning her milk, but then convicting himself by mailing a letter that Lina had written. Joan Fontaine said, Cary Grant "did kill me in the original cut, but at a preview, the audience simply refused to accept him as the murderer."
In one draft of the script, when Johnnie realizes what was in Lina's mind, he runs away until he can "find some way to pay" his debts (both financial and moral), and joins the air force under a false name. She finds out where he's stationed and proudly watches as his plane, with his nickname for her painted on it, takes off.
Alfred Hitchcock wanted an ending similar to the climax of the novel, but the studio, more concerned with Cary Grant's "heroic" image, insisted that it be changed. Writer Donald Spoto, in his biography of Hitchcock, "The Dark Side Of Genius", disputes Hitchcock's claim to have been overruled on the film's ending. Spoto claims that the first RKO treatment and memos between Hitchcock and the studio show that Hitchcock emphatically desired to make a film about a woman's fantasy life.