According to Sean King O'Grady, Lil Rel Howery chose not to know where the filming location was set and was literally blindfolded as he entered the set.
O'Grady and Howery previously collaborated in I Love My Dad (2022), which the former produced and the latter played a supporting role. They wanted to collaborate again but in a drama film rather than a comedy. Upon reading the screenplay of The Mill, O'Grady had Howery in mind for the lead role.
Sean King O'Grady said in an interview with Comic Book Resources that the screenplay of The Mill that he read for the first time was Jeffrey David Thomas's first draft.