He was already getting parts in A productions and he wasn't yet big enough to be at that "awkward" age between child and young adult actor (where you might expect him to take something just to keep working). Maybe it was to help out his father, who has a part in the movie?
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This was shortly after the ending of The Andy Griffith Show so yes, he was extremely well known. Throw in Johnny Crawford from The Rifleman and you would have expected anything better than this full- fledged disaster. Perhaps they thought it'd be better than it was. I'm not sure if either had done any films prior to this (doubtful but I didn't check) and maybe this was their first film after being in TV. I'm sure they would like to erase this one from their resume. Sometimes a movie is so bad it's good. No, this one is just plain bad!