The tradition of theatre masks goes back to the ancient Greeks, who used masks both for practical needs and dramatic heft. Masks are used in commedia dell'arte, Japanese theatre and have a long history in African culture as well. They can be beautiful or grotesque, but they are always evocative.
Traditions of Theatre Masks
In Greek drama, masks were useful devices that allowed actors to play several different characters, including those of different genders. The masks could be seen throughout the large amphitheaters in a way a face could not and were stylized so as to project the soul and emotions of the character. Stock characters were identified via masks, so that anyone in the audience could easily comprehend who was a villain, lover or king.
The improvisational commedia dell'arte that originated in Italy in the 15th century also used exaggerated masks to identify characters. Stock characters like Harlequin, his female counterpart Columbina and the swash-buckling Il Capitano always wore costumes and masks that told audiences who they were, thus setting up pleasant expectations.
In Japanese No (or Noh) theatre, masks are worn by the main actor. Dating back to the 14th century, the masks all have names and represent a variety of characters, such as women, nonhumans, children and old men. The masks are standardized as in Greek theatre. They allow the actor to use controlled body movement, even as simple as a turn of the head, to express emotion.
The "Comedy/Tragedy" Masks
The symbol of drama, the exaggerated faces of joy and sorrow are a direct descendant of Greek theatre masks. The use of masks is said to originate from the worship of the god Dionysus, who is always portrayed wearing a mask and whose sometimes violent cult of wine and celebration gives rise to exaggeration, delight and despair.
The two masks were always understood as both separate representations of the two most common forms of theatre, but also intrinsically linked in their representation of the human condition. A play can take its audience through a variety of emotions, often from one extreme to the other, and the millennia-old comedy/tragedy masks are still relevant as the depiction of that journey.
Masks and Mummery
Even in cultures that don't have organized theatre traditions, performances that incorporate masks are still part of the social fabric. The ancient tradition of mummery, derived from the Old French "momer" ("to wear a mask") was a form of greeting the new year. Masks could hide you from the evil forces that roamed the earth in the waning days of the old year.
This is also the origin of Halloween masks. Many cultures created mummery scenarios that were acted publicly at the beginning of the year, much like passion plays and pantomime. Mummery plays an important role in Thomas Hardy's The Return of the Native and is still practiced today, although now it is more like traditional theatre, rather than a sacred rite.
When Actors Play Nonhumans
Whether an actor is portraying an animal, as in the Broadway musical The Lion King, which uses traditional African animal masks as its model for performers; or a demon, as was especially popular in the middle ages, it's customary to wear a mask. Medieval plays were meant as practical lessons in avoiding sin, so masks were grotesque, thus frightening people into good behavior. Ironically, the masks were often so beautifully and intricately made, they often entranced their audiences more than disturbing them.
Modern Theatre Masks
It is rare for a modern play or musical to incorporate masks when actors are portraying humans. However, many directors re-creating traditional commedia dell'arte or Greek drama like to use masks. The masks give the actors an interesting challenge. Whereas commedia and Greek actors were specifically trained in vocal and physical techniques for characterization, modern actors are used to employing their faces when working.
While masks can be purchased or even made cheaply, if a show wants to really re-create the old effect, it's best to go through a specialized shop like Theater-Masks, which sells and custom-designs all manner of stirring theatrical masks.