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They kept them in the same clothes so that they could use the stunt material over and over again. The stunt guys wore blue and yellow, too, obviously. In fact, sometimes you can see that the diver will be wearing yellow even though in closeups, Luke is driving.

When you audition an actor, you hardly ever just have them read one thing. You want to see the range of what he can do. Odds are they just had him red for Bo to see how he handled Bo's material, as well as Luke's. According to most people's recollections, John was essentially already getting the part of Bo, so long as casting could find someone to play off of him. They probably never had Tom in mind for the part of Bo, but they did want to see how he'd handle being the more rash one.

Bo and Luke have been caught by Rosco, Luke's left hand is cuffed to Bo's right. They escape and run to Dixie. Bo: Oh great, just what I need, three hands on a steering wheel Luke: Well I'd drive but you'd have to run alongside then.