観客, 観衆 and 客席: audience names and dimmed seating lights
Compare the audience names 観客 and 観衆 with 客席 as seating space. Identify who applauds, which actors respond and what becomes dim before the performance.
観客 becoming silent and applauding actors
観客 (かんきゃく) means audience or spectator. 「幕が上がると観客は静かになった。」 means “The audience fell silent when the curtain rose.” The audience is the group that became silent when the curtain rose.
「俳優たちは観客の拍手に笑顔で応えた。」 means “The actors responded to the audience's applause with smiles.” The audience applauded and the actors responded with smiles. Those are different actions with different participants.
観衆 watching a performance or athletes
観衆 (かんしゅう) means audience or spectators. 「見事な演技に観衆が大きな拍手を送った。」 means “The audience gave the excellent performance a great round of applause.” Here the audience applauds a performance, so the word is not restricted to sports spectators by these examples.
「競技場を埋めた観衆が選手を見守った。」 means “The spectators filling the stadium watched the athletes.” The spectators watched the athletes. Its audience meaning overlaps with 観客, while the sentence identifies the particular action and object.
客席 as the lit space and the actor's view
客席 (きゃくせき) means audience seating or auditorium. 「開演前に客席の照明がゆっくり暗くなった。」 means “The lights over the audience seats slowly dimmed before the performance.” The lights over the seats became dim before the performance. This does not state that everyone had left.
「俳優は舞台から客席を見渡して深く礼をした。」 means “The actor looked across the auditorium and bowed deeply from the stage.” The actor looked across the seating space and bowed. 客席 names the space viewed, while the actor performs the bow.
Are people the thing that became dim in 開演前に客席の照明がゆっくり暗くなった?
Show the answer and why
The lights over the audience seats slowly dimmed before the performance. 観客 and 観衆 name audiences, while 客席 names the seating space; the lighting change does not establish that the entire audience left.