Thank you Michael, Keith and RAP.
This performance of John's Gospel shows it could be read in just over 2 hours. I can imagine that sort of time being available to people in the 1st century.
The Gospels do seem to be a writing down and reworking of spoken stories. They were assembled and structured in particular ways to suit particular audiences. I would like to know the extent to which they were written for Christians to use to understand their own calling, or for Christians to bring the good news to others.
Although the first Christians were Jews, the differences in the Gospels reflects differences in concerns by the time the gospels were written down. E.g. The story about which is the greatest commandment is treated differently by each of the gospel writers. Some Jewish practices are explained for people who would not have been familiar with them.
Luke-Acts is dealing with a situation and an explanation of why Christianity has become more gentile than Jewish. John's audience is in a milieu where gnostic thought is influential and I suppose deals with the question of how to bring the good news to people who have gnostic beliefs.
Matthew, with its genealogy, shows the gospel to be an continuation and completion of the Jewish faith.
Mark is said to have been written for people in Rome.
As to literacy, there are different views on how literate the people were then. Another aspect of this is that very few copies would have been available.