Spy for a Day is a 1940 British comedy thriller film directed by Mario Zampi and starring Douglas Wakefield, Paddy Browne and Jack Allen. During the First World War a British farmer is abducted by the Germans to take the place of a spy about to be executed whom he closely resembles.
Two actors from this movie, Spy for a Day, were chosen because they closely resemble each other.
The movie wouldn't work if the actors didn't share similar characteristics (hair color, height, build, etc) to represent the spy and the farmer. The computer doesn't have street-smart rationalization skills to consider that a 6 foot, stout, blonde actor couldn't play one who resembles a 5 foot, skinny, red-headed actor, for example.