Martin Esslin said, "The almost telepathic transference of images from mind to mind is the beauty and the glory of the radio play."
Also George Bernard Shaw once said about radio drama that "It provides the maximum of temptation with the maximum of opportunity." Well, he actually said it about marriage but let's not be pedantic. In radio almost anything is possible.
By far the most important piece of advice for anyone wanting to write for radio is to listen to as much as possible. Radio is a distinctive means of communication with its own limitations and possibilities and its own particular techniques and anyone who seriously wishes to sell their work to radio must make themself familiar with its existing output.
As with any other type of freelance writing, the writer must study the market. There is for instance, no single type of radio play, for the BBC's large output is broken down into distinctive series, each with their own characteristics. A play will have a far better chance of acceptance if the writer decides in the beginning if they are writing for a Radio 3 or Radio 4 audience such a Saturday night theatre or the Monday play.
The reality is the market is always right. You aren't writing for the general public, you're writing for a small handful of people whose sole opinion will decide whether your work gets produced. You don't have to know what the public wants but what a producer considers they want. But how can you find that out? One way is to buy the Radio Times, look at the radio drama listings. listen to the plays and keep a record. A record may have the following elements: date; slot; title; length; how many characters; producer; region; theme; category; plot. The purpose is to identify trends and identify producers who you feel will respond to your work favourably.
The BBC isn't afraid to tackle difficult issues these days, it's not as po-faced as it used to be. They're quite happy to broadcast difficult material. While you can get away with the odd non-sexual swearing in daytime, sexual swearwords are still referable though to the head of drama. Somebody somewhere along the line makes a decision about those words and how often and when they may be used. While the BBC isn't afraid of reflecting English as its spoken, they're not interested in gratuitously rubbing the audience up the wrong way.
These days the BBC are very concerned about people switching over. I learnt, in history at school, that people once had wirelesses and once you'd got a station you never dared moved that dial or you'd never get it back again. These days, with digital tuning, switching channels isn't a problem. There are now many more channels to switch to not just the Light, the Home and the Third as it used to be. Also radio drama is in competition with all other forms of drama including theatre and film but especially television. So when you write your plays the imperative is to keep the listener listening, capture the mood in the first minute. That doesn't mean there isn't the time for the slow-build but it should still have something that people can get a hook on from the start. As with all drama the story has to start early.
Radio does some things much better than any other medium and they're always interested in plays that can be done in no other medium. Consider plays that not only can't be done in any other medium but are impossible to do the ironing to.
1: Introduction
2: BBC Birmingham Workshop