Replaced for this movie was the canine actor who had played Asta since the first Thin Man film. The original dog, Skippy, outgrew the part.
This movie was to begin production in 1942, but Myrna Loy refused the part. Instead, she went to New York where she married car rental heir John Hertz Jr. and worked for the Red Cross war-relief effort. The movie almost began shooting with actress Irene Dunne as Nora Charles.
Liberal drinking of alcohol, a mainstay of the first four "Thin Man" movies, was curtailed for this movie due to wartime liquor rationing.
Nick is lying in a hammock 22 minutes into the movie, with a copy of "Nick Carter, Detective" magazine. This issue is Volume 7, #3 (May 1936), thus making it 8 years old at the time of filming. The use of this magazine is an homage to the long-time pulp detective Nick Carter, from whom Nick Charles got his own first name and surname initial.
Wartime audiences would have been very familiar with the packed passenger train shown in the film, including the standing room only passengers. During the war, trains moving troops and war materials had priority over civilians traveling by rail for non-essential reasons.