The Discovery article describing a collection of masks recently "rediscovered" from Pompeii pointed out that some of the masks had closed mouths indicating they were used as models for the mask craftsman. However, the masks could have been worn by dancers in a pantomine, who wore masks with closed mouths because they did not speak but performed using only expressive gestures.
"...the most popular genres were unquestionably mime and pantomime, which sought to please the audience. Here even nudatio mimarum (a sort of striptease) was sometimes staged or, more surprising yet, the reenaction of real executions and torture. Mime was based on action and performed without masks. It was the only type of performance in which women played the female roles."
The number of days devoted annually to [theatrical] games gradually increased over time. At the end of the third century BC there were probably twelve at the most and, yet, at the beginning of the Empire, there were already 56 consecrated to theatre performances, reaching 100 by the mid-fourth century AD - The Roman Theater: Staging the Performance
A set of 15 mysterious life-size masks, reminiscent of ancient Roman drama, have been rediscovered in Pompeii after being forgotten for more than two centuries, according to Italian archaeologists who have shown them for the first time at an exhibition in Naples, Italy.
Made of plaster, the rather heavy masks were unearthed in 1749 in Pompeii during the excavations promoted by King Charles of Bourbon. They were deposited, along with many other artifacts, in the Royal Palace of Portici, a town on the Bay of Naples.
"Two masks show letters in the space usually reserved to the mouth. While the meaning of one is incomprehensible, on the other we can clearly read the word 'Buco,'" Borriello said. The word refers to Buccus, a stock character from the earliest form of Italian farce, known as fabula Atellana.
Deriving its name from the town of Atella in the southern Campania region, the fabula Atellana was a form of entertainment widely popular from the second century B.C. to the fourth century A.D. Basically a form of improvised farce, it used masked actors, stock characters and conventional plots. - More: Discovery News
Atellan farces also relied on the physical comedy of slapstick and burlesque.
By the early Imperial period, Atellan farces were no longer improvisational, but scripted performances.
Livy describes the Atellan Farces and the names for the actors (histriones) in section 7.2. of his History of Rome, where he says the Romans first performed them to try to fight a pestilence (in 363 B.C., according to Richard C. Beacham in The Roman Theatre and Its Audience). - About.com