Perhaps the Romans are best known for their Amphitheatres rather than their theatres. In fact the Amphitheatre was a double theatre back to back making it into an oval “arena” such as the Colosseum.
Ancient Roman theatre was very much inherited from the Greeks when the Romans came into contact with Greek culture during their invasion of southern Italian city states. The influx of Greek slaves who might have been actors or even playwrights gradually brought an appreciation for drama to Roman culture although it could hardly beat the appeal of the Circus and Amphitheatres such as the Colosseum.
According to Greek tradition, actors on the stage would wear large masks which often had large wide-open mouths: probably to project their voices better as well as to render their character’s features most easily seen.
This drawing made from a clay figurine found at Pompeii, which was historically heavily influenced by Greece, gives an idea.