How to Decide on the Optimal Cast Size for your Scripts

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microphone by Miyukiko © 2013
microphone by Miyukiko © 2013

Ultimately, the cast you need is the cast necessary to tell your story. That said, there are a number of practical considerations that should be taken into account when writing for audio.


This week I’m throwing in some quick thoughts regarding how many characters we can populate our stories with.

There are a couple of small issues I believe are worth thinking about when introducing characters. And there are at least two limits that we come up against when introducing regular or recurring characters.

The first relates to short term memory. Human short term memory has an upper limit of about 7 +/- 1 items that can be held in mind. That means, we can only keep track of about 6 to 8 new characters (at the outside) in any given scene before we will run up against this limitation in our brain and start thinking “who was that again”.

Four or five, generally speaking, is a pretty safe number.


When the characters are established enough that we don’t need to keep them in short term memory any longer (we’ve gotten to know them) the brain will chunk them as a group and allow us to add another 5 to 7 characters to our short term store. Once they are established enough that we don’t need to rely on short term memory to remember them as well, we can treat them as a second group and, theoretically, add a further 4 to 6 characters etc. Notice that there is a law of diminishing returns at work here and, for another reason (below), the upper limit on recurring characters may be struck much sooner.

The second limit is one based on our ability to distinguish between human voices. In the days of OTR casts were kept to around 6-8 in order to keep the voices distinctive and easily differentiated. They would cast voices according to 7 easily distinguished voice types.

  • Bass – elderly male,
  • Contralto – elderly woman,
  • Baritone – leading man,
  • Mezzo-soprano – leading woman,
  • Tenor – juvenile male, Soprano – ingenue,
  • and Trebble – child)

This didn’t mean that the main cast was all there was, but attempts were made to avoid castng recurring characters with the same vocal tone, and when introducing bit and throw-away parts, trying to differentiate the voices of those actors from those of the main cast.

Accents would allow for further differentiation of characters but generally speaking, scenes were designed to have an upper limit of no more than six or seven characters in them at any one time. This meant that no one was likely to become confused by the presence of similar voices in conversation (something that happens quite easily if you happen to cast actors with similar voices).

You can extend this limit by preventing actors of similar vocal tone from interacting and/or by having certain characters limited by context so that they remain identifiable (Joe, who sounds a bit like Pete, is always at the drugstore for example), but I suspect with a large cast of recurring characters this will place a limit on how many the audience can follow without confusion much sooner than the issues around short term memory will.


The more characters added to any given scene, the more lines they all have to have just to stay alive in the minds of the audience. In listening to more than a few shows I’ve found myself going “oh yeah, I forgot he/she was there” when they’ve suddenly spoken up after a long silence.

Do you have any other advice regarding casting? Tell us in the comments.

This article is © 2017 by Philip Craig Robotham – all rights reserved.

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