Avoid characters, themes and situations that have recently been done.
Radio is not like film, where a hit will spawn a host of imitators. A successful sitcom series on Radio 4 guarantees the network won't want anything similar for some time after.
Avoid trying to be too topical, especially given that the length of the commissioning process will make a flash-in-the-pan topic date quickly. Stories and situations that seem to resurface frequently include history, space, the media, parallel universes, school reunions, and the afterlife.
The idea has to be one that genuinely excites you, and that you have some genuine reason for writing. If someone asks why you want to write the idea and your only response is "because it hasn't been done," the chances are it will come across as uninspired. Bring your own unique comic insight into a particular situation or world, and you can probably only do that if you really care about it.
Having too narrow a theme and overworking it can be as dangerous as having no focus at all. Many new writers stop at one idea and overwork it - try to work in sub-themes as well as a main theme.
You don't have to think of an original environment. It's more important to give us a new perspective on something familiar, or a fresh style. Surreal or novel ideas are as commonplace as ones set around a sofa.
All your characters should have an original slant, comic potential and mileage. They need to have a comic flaw or two - some weakness that keeps getting them into trouble. They should interact with each other to create comedy, but should also remain believable. Characters should be likable, even if they aren't necessarily 'nice'. Comic empathy comes through making your characters suffer for their mistakes, yet be making them always somehow unaware of their faults.
Telling stories is important. The main story should relate to your main over-riding theme. If your comedy is about how ambition can lead to disaster, then the main plot should demonstrate that point. But it's good to have several storylines and spread the comic possibilities - and they do not necessarily have to neatly tie-up or connect.
Make sure the humour is driven by the characters and stories, and not just about funny lines put into character's mouths. Avoid characters sniping at each other 'in a funny way'. Many writers assume that writing comedy for radio means just writing gags. It's worth limiting the number of formulaic lines - eg "That's like a cross between..." or "That's about as healthy as..." or "I haven't seen anything as bad as that since..." Great characters are funny to an audience not because they crack jokes or make witty repartee but because of the things they do, choices they take, mistakes they make.
Avoid factual exposition. The audience very rarely needs to know much about a character's past or how they came to be in the situation they're in. How much do you know about the pasts of the Steptoes, Basil Fawlty, Del Boy or Blackadder?
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