I would say that the most important thing is to have interesting characters who connect with the audience. Radio drama really strips it down to that most essential part. They have to grasp the imagination of the audience, though good vocal casting helps and chemistry between the actors is just as important on radio as on screen.
I think radio drama should be larger than life - ghosts, vampires, castles, alien worlds, submarines, and so on. Radio can transport listeners absolutely anywhere, and I like to hear shows that take advantage of that.
I listen to the Prairie Home Companion on NPR all the time. I love it. I would like something like a mystery or a comedy to really keep me listening. Maybe a funny mystery...
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