As with characterisation, radio dialogue is governed by a number of principles that apply to dialogue in general, in addition to some radio-specific principles. Let us begin with the general and progress to the specific.

WHAT MAKES A GOOD SPEECH?

• Perhaps it consists of one or more good sentences, logically organised.
• It is relevant to the plot.
• It is true to the spoken word, i.e. it sounds like real speech rather than written text being read aloud.
• Perhaps it is short, maintaining listeners’ interest and keeping the piece moving along.
• If it consists of more than a few sentences it is calculated to achieve a specific purpose. Examples of "good reasons" for longer speeches include:
  — scene-setting,
  — making and reinforcing a crucial point in the plot,
  — marking a key stage in the revelation/development of a character,
  — having a calculated effect on the audience (e.g. rousing, hypnotising, evoking empathy),
  — providing a summary of past or planned events and
  — acting as a bridge to mark the passage of time.
• Perhaps it facilitates the task of listening by exploring or reiterating complex words, ideas or aspects of the plot.
• Perhaps it makes provision for effective use of silence.
• Perhaps it refers to a previous speech or scene, giving a conflicting account which reflects the speaker’s particular point of view.
• Perhaps it contains inconsistencies that reflect the character of the speaker.

WHAT MAKES GOOD DIALOGUE?

• Each character’s speech prompts questions/issues in the other’s mind, directly triggering a response, or perhaps causing the next speaker to change tack.
• Each speech connects with the previous one, unless of course the speaker deliberately deviates from the established line of argument.
• Perhaps a speech challenges or contradicts a previous speech.
• Perhaps the characters’ contributions to the dialogue reveal conflicting motives or objectives.
• It reveals some new aspect of at least one of the characters involved, and ideally all of them.
• Perhaps it marks a significant development in the plot.
• Perhaps it resolves an outstanding issue and/or gives rise to a further issue to be resolved.
• It involves well timed interaction between the characters.
• It exposes aspects of character that might not be apparent from descriptive text.

WHAT ADDITIONAL OPPORTUNITIES DOES RADIO DIALOGUE OFFER?

• The absence of any visual dimension allows the writer to engage in flights of fancy that might not be so credible in any other medium (consider The Goon Show, for example).
• The writer can use (non vocal) sounds in place of words when appropriate, thereby increasing the conciseness of the text.
• Characters may communicate across barriers of time and space more freely than in, say, theatre.
• The writer can exercise a greater degree of control over the sounds of speech (through brief instructions to the actors) than, say, a novelist, whose written words may be heard differently in the imaginations of respective readers.

WHAT ADDITIONAL CONSTRAINTS APPLY TO RADIO DIALOGUE?

• Dialogue must convey visual clues that cannot be communicated through non-vocal sound, but without making the characters speak unnaturally.
• Radio characters must express themselves in particularly concise terms, in view of the compressed timescales of most radio slots.
• Dialogue must always move at a fairly rapid pace, in view of the all-important factor of keeping listeners' attention.
• Characters stylistic "voices" must be clearly differentiated, particularly if listeners might have any difficulty identifying which character is speaking. For example, if there are many characters in the play, the scope for categorical differentiation between voices may be limited accordingly.