Alec Guinness did not enjoy doing this movie. It required much more time than he expected, with the need of wires and a harness for his floating character. He suffered a double-hernia that required surgery to repair.
Alec Guinness' big musical number was cut from the film, although the lead-in remains intact. It was called "Make the Most of This Life" and was restored when "Scrooge" was adapted into a stage musical with Jon Pertwee as Marley and Anthony Newley as Scrooge.
Although the music was composed by Leslie Bricusse and nominated for two Academy Awards, Bricusse could not write music. He would dictate lyrics and melody to music supervisor Ian Fraser who would transcribe and arrange them for Scrooge's score. Bricusse did so on many other movies to much acclaim.
Richard Harris rejected the role of Scrooge. Rex Harrison agreed to play the part, but had to back out due to a commitment to a difficult play. (Harrison was also having an affair with Harris' then-wife, who he would later marry.) Albert Finney, who had been offered the role before Harrison but had initially rejected it, reconsidered once he read the script and asked for the role. (He was a business associate of Michael Medwin, the co-writer who played his nephew in the film.)
Kay Walsh, who plays Mrs. Fezziwig, played Nancy in David Lean's 1948 version of Dickens's "Oliver Twist". She also collaborated on the screenplay for Lean's 1946 version of Dickens's "Great Expectations". Ronald Neame, who directed the 1970 "Scrooge", produced the 1948 "Oliver Twist".
Apparently this film unlike the book and most other film versions takes place in 1860 instead of 1843. This is revealed after the Ghost of Christmas Present asks what year it is and Scrooge replies "1860". This means as a result the film is set 17 years later and as Marley had been dead for seven years his death was in 1853 rather than 1836.
After Scrooge drinks some milk offered by the Ghost of Christmas Present he asks if he can have some more. This line is almost very similar to the famous quote Oliver Twist says when he asks for more gruel.