Well it's not based on a movie, but I once ran an adventure based on an episode of the Dad's Army TV series that I remember being good fun.
No-one realized where the plot and characters came from but that wasn't very surprising, since the location was shifted from Walmington-on-Sea on the English coastline to a town in a fantasy campaign that's basically a potage of Indian/Chinese/Mesoamerican folklore, media tropes and history. It's not like I had the village bank manager shouting out "Don't tell him, Pike!", which would be a dead giveaway.
However I find literature a far more useful source of inspiration for RPG sessions.