Besides the vocal aspect, here are the top three things I look for before deciding what vocal warm up to do.
- Time of day: If rehearsal starts in the morning, I spend a lot of time on gentle warm ups, such as: 123454321 on "n," scale up and down on lip trills, 13531 on "ah," 54321 on "zi-i-i-i-i," 16765654541 on "ee."
I warm up the middle range, and start by going to only about the e a tenth above the middle c before going down by half step.
- Vocal health of the cast: This really depends on how (un)healthy the cast is. If a few people are congested, I have the cast block one nostril, and sing through the other one on "n" 54321, going up to no more than d (11th above middle c), before switching nostril.
When cast members have done too much belting too suddenly, I emphasize warm ups on mixing. If they're feeling tense from the rehearsal process, I have them jump on the word "love" for "I love to sing" on 18531.
- Energy level: When a cast spends all day in cue-to-cue or in tech and gets into relaxed mode, it can be hard for them to transition back into the energy necessary to do a run.
I do more of quick warm ups, with emphasis on diction. belt and mix, such as: 54321 "how are you," 13531 "gna gna gna," 132435421 "bee dee bee dee," 135---31 "ay" (with no diphtong, IPA epsilon).
Picture from http://www.vocaltrainingsecrets.com/vocal-warm-up/