It is believed that the use of masks is related to the cult of the ancestors, which considered dancers the interpreters of the gods. Topeng performances open with a series of non-speaking masked characters which may not be related to the story to be performed. These traditional masks often include Topeng Manis (a refined hero), Topeng Kras (a martial, authoritarian character), and Topeng Tua (an old man who may joke and draw-out the audience).
The story is narrated from a Penasar, a jawless half-mask that enables the actor to speak most clearly. In group topeng, there are usually two penasars providing two points of view. The performance alternates between speaking and non-speaking characters, and can include dance and fight sequences as well as special effects (sometimes provided by the gamelan). It is almost always wrapped-up by a series of comic characters introducing their own views. The narrators and comic characters frequently break western conventions of storytelling by including current events or local gossip to get a laugh.
Source : http://en.wikipedia.org/wiki/Topeng_dance
Picture from Gunawan Kartapranata