Often the original stars reprised their roles, but sometimes they were unable to do so. When they decided to run Casablanca Bogart was overseas entertaining the troops. Alan Ladd stood in for him. Another interesting thing with the Lux Radio Theater is that the studio system wasn't enforced, so it was possible to get actors working together across studio lines, which was quite revolutionary.
About a hundred of these recordings survive, and you can download them and listen to them, interspersed with the original advertisements for Lux soap. Absolutely fascinating; there are some great ones such as the aforementioned Casablanca, but other favourites of mine include The day the earth stood still, The ghost and Mrs Muir, Mrs Miniver, The Maltese Falcon, After the Thin Man, A farewell to arms and Miracle on 34th Street.
These are fantastic recordings, the adverts, the stars, the original crackles and pops... if you've any interest in Hollywood recordings, you'll love listening to these.