Michael Hordern was not on set when the "Marley's Ghost" segment was filmed; he was added in later through the use of an optical printer. He only appears together with Alastair Sim in the two scenes at the end of the "Ghost of Christmas Past" sequence, the latter of the two being the scene where Jacob Marley dies. This was also true of Michael Dolan, who played the Spirit of Christmas Past; he never actually played any scenes on the set with Sim.
In the novella, the Spirit of Christmas Past carries an extinguisher, a small funnel which was used to put out candles. This was eliminated for the movie version, although the Spirit does appear more or less solid, depending on the scene, to correspond with the description in the book.
Although this film is widely regarded as the best film version of Charles Dickens' story, it is the only one which omits Scrooge's famous line: "If I could work my will, every idiot who goes about with 'Merry Christmas' on his lips should be boiled in his own pudding and buried with a stake of holly through his heart". Alastair Sim would eventually get a chance to say it however, when he reprised his role in the animated A Christmas Carol which also featured Michael Hordern returning as Marley.
The word "humbug" is misunderstood by many people, which is a pity since the word provides a key insight into Scrooge's hatred of Christmas. The word "humbug" describes deceitful efforts to fool people by pretending to a fake loftiness or false sincerity. So when Scrooge calls Christmas a humbug, he is claiming that people only pretend to charity and kindness in an scoundrel effort to delude him, each other, and themselves. In Scrooge's eyes, he is the one man honest enough to admit that no one really cares about anyone else, so for him, every wish for a Merry Christmas is one more deceitful effort to fool him and take advantage of him. This is a man who has turned to profit because he honestly believes everyone else will someday betray him or abandon him the moment he trusts them.
The song that Mr. Jorkin whistles after offering Scrooge a job is "The Lincolnshire Poacher", wherein a poacher sings how much he loves unlawfully entering property and hunting and trapping the game there. Poaching can also refer to the hardball business practice of hiring an employee away from a competitor. Jorkins is trying to do exactly that with Scrooge. In Jorkins' next scene, he has succeeded.
The trivia item below may give away important plot points.
Changes to the screenplay from the Charles Dickens novella were made, mostly in the Christmas Past sequence. Among them being: - Reversing the birth order of Scrooge and his sister, so as to add that Scrooge's mother died giving birth to him. - Creating a character named "Mr. Jorkin", who does not appear in the book. - Flash-backs of several incidents in Scrooge's past (e.g. his sister's death, meeting Jacob Marley, taking over Feziwig's Warehouse, and Marley's death) which do not appear in the book.