In the game Grim Fandango, we find out that the protagonist, Manuael Calavera, is working off a debt to the powers that be. Presumably, he committed a sin in his life which meant he had to work for the DoD before he would be allowed to make his 4-year journey to the 9th underworld.
Is it ever explained, in the game or out (interviews, publications etc.) what it was that Manny did?
It’s a great question, but I’m not sure there’s a definitive answer, other than “we don’t know”.
According to the character’s page on the Grim Fandango wikia:
Almost nothing is known about Manny’s time in the Land of the Living. Periodically throughout the game, Manny states he doesn’t know what sin he committed [emphasis mine] that got him stuck at the Department of Death, but that he is willing to work his time off regardless. In Year 1 he says that he has none that he wishes to visit during the Day of the Dead.
My best guess – it’s left intentionally unanswered as it gives him an air of tragic mystery (even he doesn’t know) and is actually irrelevant to the story anyway.