Composite warm-up facts, in rambly paragraph form: If I'm on as Elphaba, I warm up in the shower. Every time. Whether or not I actually wash myself is variable. Elphaba and Glinda's dressing rooms share a wall; we can usually hear each other warming up, and have an elaborate Morse-like code whereby words are pitches and vowel sounds are punctuation.* During WICKED's opening number, while I'm still backstage I usually sing along with Glinda's solo ("Let us be glaaaaad...") as I hear it come through the monitor (every dressing room has a monitor through which the show is broadcast, live). In the backstage area downstairs there are no rehearsal spaces, and many of the "walls" that separate the dressing rooms are more like partitions, as they do not reach all the way to the ceiling. This makes vocal warmups well-nigh impossible at the theater! Most cast members end up doing their warmups at home before arriving at the theater, or some sneak off to nearby stairwells to get in some final puff-hums before curtain. The name of the game is resourcefulness!
*The last part of this sentence is untrue.