If the subject is in the middle of an important story, let him finish it, then have him repeat it. Take notes for the script on location. Write down or record notes on how people look, what they're wearing, what the environment looks like, etc. All notes should be done on location. Help the interviewee be more descriptive.
Emotional content works very well on radio.
Don't be afraid to rerecord. Rerecord it until you're completely happy with the reading. Make it a practice never to turn off or pause your tape recorder. Leave it running as much as possible. When you hear something dynamite that you have a strong feeling will be used in the final piece, stick with it.