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Principles of writing radio drama.

If you're new to writing for radio, there's a great book
called THE WAY TO WRITE RADIO DRAMA by Bill Ash (published by Elm Tree Books).
However, here are a few suggestions to get you started:
First of all, the way you choose to present your script is really important
if you want the reader to enjoy your play and to concentrate fully on it. Here
are a few things which can make reading difficult:
- Plays presented on brightly coloured paper
- Plays printed with expiring cartridges so that the type is so faint you
have to squint at it
- Plays printed on continuous listing paper with the content of each page
dropping down on to the next one (one of our readers likened this to reading a
toilet roll)
- Plays typed in unusual fonts which are really difficult to read, or typed
in very large or very small fonts
- Plays typed in such a way that there's no differentiation between character
names, sound effects and dialogue
- Plays hand written in pencil
A common mistake is to abbreviate characters' names down to a single initial
so the reader has to keep stopping to think who's speaking. The best way to
present a script is not so that it will stand out from the competition, but just
the opposite - if it's neatly typed, double-spaced, with the characters' names
in capitals down the left margin and sound effects or music described in
italics, it'll be so easy to read, the reader will find it effortless. Then they
can really concentrate on the content of the play. 
As for the content of the play - radio drama is very different from theatre
and television. We often get scripts which say 'written for the stage but I
thought it might do for radio' on the front cover. It usually doesn't. Stage
directions like 'puts glasses on' or 'bends over and picks up a piece of paper'
aren't going to be easy to get across to a radio audience. It has to be in the
dialogue or sound effects, otherwise it will be lost. Stage plays can be adapted
for radio, but not without a great deal of attention to detail. 
There are no limits on the imaginative potential of the dramatist and there
are no limits on how the radio dramatist can express that potential. The sound
medium is free of all the physical and practical limitations of the stage and
film set. A radio play can move through any dimension of time. It can move to
any location. It can voice metaphysical, surrealist and subconscious feelings
and images.
The radio dramatist needs a creative interaction with the listener, and the
ability of the radio dramatist to create a unique world in the listener's
imagination is a special quality generally denied to TV viewers or theatregoers.
Theatre, film and TV plays paint the colours for the audience, whereas the
pictures in the mind of the radio listener are individual to that person alone.
This gives radio a special intimacy.
Radio is very good at dramatising what people are thinking. The contrast
between what people say and what they think can be shown very effectively on
radio. 'Interior thought' is a convention which is special to the radio medium.
In radio the listener can be instantly transported inside the head of a
character and can hear those secret, private thoughts that are often better left
unsaid. Radio drama has been described as 'The Theatre of the Mind'. The
potential conflict between exterior dialogue and speech and interior thought
creates a special  ground of conflict which is a fertile area for the creative
radio playwright.
The key to writing successful plays for radio is to  realise that the
listener can only understand what is going on by what he or she hears. The
physical environment and the appearance of the characters depends on what they
say and the images created in the listener's imagination by words and sounds
and/or music.
Radio drama has only one chance to be successful. A dissatisfied listener
only has to twiddle the radio dial, or turn the switch off altogether and the
playwright has lost the audience. It's important to bear in mind that a radio
playwright can't afford to bore or confuse the listener.
All good writing requires a strong element of honesty and integrity from the
writer. The great dramatists strive to express a truth about human experience or
human nature. This means that superficiality, pretentiousness and ostentation
are quickly recognised by the listener and will normally lead to failure when
the listener switches off.
Good luck with your writing!
CLICK HERE FOR 'PRINCIPLES OF
WRITING RADIO DRAMA' BY TIM CROOK