Not in Play: From the previous round, I would challenge whether the Thin Man was a McGuffin, since his being a person rather than say a diamond ring, or a painting was crucial to the plot.
an unfinished and unreleased 1972 Swedish-French film directed by and starring Jerry Lewis. The film was met with controversy regarding its premise and content, which deals with a circus clown imprisoned in a Nazi concentration/extermination camp. Lewis himself repeatedly insisted that the film would never be released, but in 2015 he donated an incomplete copy to the Library of Congresa with the stipulation that it was not to be made available before June 2024. Even so, outstanding copyright issues have prevented its release.
But (spoiler alert) the whole story revolves around a mysterious Thin Man. At first he’s suspected of being a murderer, but ends up being the murder victim. The story revolves around his identity and his role in the murder. And, having been dead from the beginning, he really doesn’t contribute anything on his own.