Useful background here. 

Also here: Skenotheke: Images of the Ancient Stage

and here (more theater images from around Greece); and here (excellent images)

Archaeopaedia articles on Theater (from Stanford University); includes discussion of costume, mask, and theater history, with some bibliography.

Theater of Dionysus:

Theater of Dionysus Factoids

Seated 15-20,000 (!), i.e., 5-7% of Athens' population (ca. 310,000). Slaves and metics (resident aliens) were also allowed entrance, as were, probably, women. A public festival fund (theorikon) subsidized tickets for all citizens, esp. the poorer classes.

Detail of ancient theater:

Videos:

Ancient Greece:Greek Drama w/C. Pelling

Delos:

Epidaurus (4th c. BCE; expanded under Rome):

click to enlarge!

Seats 15,000. Read here on the amazing acoustics of the theater. (More on acoustics here.)

Areal view of Epidaurus

Theater Functions

Ideal reconstruction (Roman theater):

Masks

Greek

Roman terracotta copy of Greek tragic mask

Roman terracotta copy of tragic mask from British Museum (details

here) 

Actor contemplates mask, late 4th c. BCE

Roman

Poses were significant

Two statues (Roman), one comic, one tragic