As someone who was highly involved in musical theatre in high school and college (my minor actually is vocal performance), I've always been a sucker for Broadway musical soundtracks.

Les Miserables was the first film version of a Broadway musical to actually use live vocals in the movie. Normally, the actors would record the soundtrack in studio and then lip sync during the scenes. Les Miserables broke barries by doing this and they did it with the actors wired with a small almost invisible ear piece in one ear with a piano instrumental. Later in production, a full orchestra recorded in studio and was added to the actors vocals. It was done like an actual stage show, where the pit (orchestra) follows the vocalists instead of the singer following the instrumentals.