This is very important. I generally find four characters are the max that the radio audience can handle without confusion settling in as to who is talking. It's easy, Col. Frothingham. Try to select scenes that have some action, otherwise you risk lulling the listener to the point of tune-out. A wisely used narrator allows you to save a lot of time in describing characters, setting and past action.
Many writers hate the narrator.
Just use it effectively. I generally have narrators speak at the beginning of a scene and no more. I avoid having them speak in the middle of a scene because it yanks the listener out of the drama and breaks the spell we're trying to weave. I'm upset over the recent turn of events. There, there, John.