Mystery plays were a product of the fifteenth century (1400s) and were not the same as miracle plays, which emphasized intervention into human lives. Mystery plays, in contrast, depicted events as recorded in Scripture. If I understand your question, this was one of the ways that mystery plays were staged: (1) It was required that mystery plays stay true to Scripture or stories of saints' lives. This leads to the second way in which mystery plays were staged. (2) They had to be under one of the three mystery cycles: Old Testament, New Testament, Saint's Lives. This means that narratives from the Old Testament were turned into mystery plays, as were narratives from the New Testament, including, most famously, the passion of Christ. Additionally, Church Saints were memorialized and honored in mystery plays that told their life stories. Within this guideline, though, there was the inclusion of comedic scenes that featured what is referred to as buffoonery.
The third way in which mysteries were staged was, in some cases, an unfortunate one: (3) While the productions had simple and generalized stage settings, like a few stationary trees, the production had to last as long as the event being told. This was especially significant in the New Testament cycle passion plays. For instance, when the crucifixion was acted, it lasted as long as the Biblical account indicates it lasted; when Judas is being forsaken, he hangs as long as the account indicates he does. In a couple of instances, referenced by the New Advent Catholic Encyclopedia, actors lost or nearly lost their lives through complying with this staging requirement:
Moreover, the scene of the crucifixion had to last as long as it did in reality. It is related that in 1437 the curé Nicolle, who was playing the part of Christ at Metz, was on the point of dying on the cross, and had to be revived in haste. During the same representation another priest, Jehan de Missey, who was playing the part of Judas, remained hanging for so long that his heart failed and he had to be cut down and borne away.
An example of a mystery play is The Second Shepherds' Play, which is in the New Testament cycle and an antecedent to a passion play. While it covers the serious subject matter of the upcoming birth of Jesus and the role of the shepherds in the birth story, it is laced with comedic scenes of buffoonery between the shepherds. It was staged anywhere from the middle of small town streets to the great halls of nobles' castles, with stage setting scenery that accommodated any location, though this mystery may have had two sets: Mark's house and the manger.