The word "humbug" provides insight into Ebenezer Scrooge's hatred of Christmas, as it describes deceitful efforts to fool people by pretending to a fake loftiness or false sincerity. Therefore, when Scrooge calls Christmas a humbug, he is claiming that people only pretend to be charitable and kind in an effort to delude him, each other, and themselves. In Scrooge's eyes, he is the one man who is honest enough to admit that no one really cares about anyone else, so (to 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.
Changes to the screenplay from the Charles Dickens book were made, mostly in the Christmas Past sequence. Among these changes are: 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" and flashbacks of several incidents in Scrooge's past (his sister's death, meeting Jacob Marley, taking over Fezziwig's warehouse, meeting Bob Cratchit, and Marley's death) which do not appear in the book.
The song that Mr. Jorkin whistles after offering Ebenezer Scrooge a job is "The Lincolnshire Poacher," wherein a poacher sings how much he loves unlawfully entering property and trapping game there. Poaching can also be the practice of hiring an employee away from a competitor, which is what Mr. Jorkin is doing with Scrooge.
Although this movie is widely regarded as the best version of Charles Dickens' story, it is the only one which omits Ebenezer 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 Un chant de noel (1971), which also featured Sir Michael Hordern returning as Jacob Marley.
Although the word "Scrooge" means a stingy person now, in Charles Dickens' time, the word was a slang term meaning "to squeeze."