Both times I saw Phantom on Broadway, and the time I saw the original production's national tour, the audiences were always very well-behaved. So, to give you a non-needlessly snarky answer, people tend to really love it when the chandelier falls at the end of act 1. Depending on how well it's sung, folks also love to give a healthy round of applause for Music of the Night. Phantom is one of my all-time favorites, so I hope you enjoy the show, too!
"Was uns befreit, das muss stärker sein als wir es sind." -Tanz der Vampire