The suspense shows of the 1940s used small ensembles and thus are fairly easy to reverse-engineer for their orchestrations. I think The Adventures of Philip Marlowe is one of the best scored radio programs ever. You can pick up a lot by imitating the greats. Melodies can pull a listener's attention away from the dialogue. Interesting melodies are only good for bridges or program themes.
Radio drama requires minimal melodies.
This melodic limitation often pulls the rug from under musicians learning to score. If you are composing a bed, keep the musical elements underneath the dialogue or the mixer will do it for you by turning down the music volume and nobody will hear your score. You thus have failed to serve the drama, which is a composer's number one function. Each one has a slightly different shade of mood. Make note of which chord delivers which effect.