A mummers play was a kind of folk play with a long tradition in at least Britain. According to the Fifth Doctor, the plays were "an oral tradition, passed down since the Middle Ages". Performers were generally not professional, but drawn from the local populace. Because the plays were passed down generationally, the audience generally knew the script as well as the performers. Indeed, roles could simply be awarded to audience members while the play was running, without too much fear that such an abrupt handover would hinder the performance.
A particular Christmas mummers play was passed down in Stockbridge, England for centuries, which the Fifth Doctor and Nyssa witnessed and even performed in on Boxing Day 1899. It was based on historical events involving St George slaying the dragon, and the Doctor was surprised to discover that he was a character in the play. This led him to conclude that he and Nyssa were actually part of the historical events — but not before he was forced to play himself to an audience only too prepared to dislike his interpretation of the part. (AUDIO: Castle of Fear)
In 1902, a ship crashed containing Addos. These Addos witnessed a mummers play before they went into emergency hibernation. When they awoke in 2014, they tried to repair their ship by recreating the last thing they saw an audience enjoy - thus turning people into animal hybrids, repeating the words "The revel begins!" which were the words spoken during the mummers play. (PROSE: Behind You).