As far as I know, SLJ's casting in the prequels was down to an interview he gave on a british TV show called TFI Friday (thank fuck it's friday) which aired every friday night in the mid nineties. It was probably the biggest chat show of the time, and pretty much summed up nineties britain. Anyway, Samuel L was on the show and he said he was asked what his dream role was and he said he'd kill to be in the new star wars movies "I'll do anything, I'll play Luke Skywalker's slave if I have to" and the rest is history.