In Les Miserables, I understand all music numbers (every scene, that is) were recorded live on set with the actors leading an accompanying pianist off-site. How then could they shoot the One Day More number? This piece consists of many locations and many intersecting melodic lines from many different characters. Is it possible they shot all scenes simultaneously on separate portions of the stage and all vocal performances were monitored through every performers' in-ear system?
I would guess that they could either:
(Do not forget that they would have rehersed these songs scores of times and would know them inside out).