Short answers, pulled from the story.
The Theatre of Pompey was constructed in 55 BC, ending a moratorium on permanent stone structures. Before this date, temporary wooden stages served the public for short periods during festivals.
Concrete became the primary building material for Roman theatres across the Empire from Spain to the Middle East. This substance allowed construction anywhere without relying on natural slopes or earthen foundations.
The scaenae frons rose high behind the stage floor and supported columns to direct sound toward the audience seated in the cavea. This wall helped enhance natural acoustics through semicircular forms crafted by Roman designers.
Political or financial hurdles could halt construction before completion, leaving some Roman theatres with unfinished states. These incomplete buildings serve as archaeological evidence for challenges faced during the transition from wood to stone.
The Roman Theatre of Orange in modern France remains a good example of a classic Roman theatre today. Its indented scaenae frons reflects Western Roman theatre designs stripped of ornamental stone columns and statues.