The audience and its expectations
For the reader of Shakespeare, one of the most important things to understand about the mystery plays is the way that they shaped the expectations of the audience that was later to watch plays by Shakespeare and his contemporaries.
Unlike the plays of Rome, with their careful "unities," the mystery plays took place over a huge span of time--from Creation to the Day of Judgment. Place too was continually changing, from Heaven to Hell to Earth. Even within the separate playlets time and place varied, from the interior to the exterior of the Ark, and from the beginning to the end of the Flood, for example.
The audience also expected to see the action on stage-- the crucifixion, or the murder of Abel--rather than having a Nuntius (or messenger) announce offstage violence.