Every time I see a band start messing about at the end of a tune discussing what to do next I always think A**hats, and tend to leave. I'm still su-prized at the amount of times I see this. The only time you'll get a pass is if it's an unexpected call up and someone is invited to join. There will be chatter but keep it fast.
Being absolutely sarcastic here but there's this thing called a set list? Have you tried one? Tends to provide focus.
Look it's simple there's a difference in rehearsals, and here's the formula I always use. There's the lets just practice, have fun, shoot some breeze types which are the most common. The there's the second type is the run up to a show. At the start of rehearsal I give fair warning and say that the show starts in 5 minutes, pretend you're on stage and it really counts, don't stop unless it's planned, tune the SECOND the songs over if you drift, but go at least 4 tunes before doing so. Singer talks here, keys reprogram there etc, and account for same in the set. I will time this to ensure that we hit the times. Yes I have used a stop watch in some cases, I will even go as far as a simulated load and setup if I think it's warranted, that way everything's slick and on the numbers.
One or two of those will demo the what time changes are required, where to have vox cover cover for say drop D tunings and such. You then group it all together so that once in drop you stay there. Run it again and look for lag. It'll get to be a science rather quickly and those that seem to fall behind at first will quickly get caught up in the charge. Pretty soon you can just do this without thinking.
Then you walk out on stage, slicker than snot and the next thing you know you're in the next tier, which I assure you will NOT happen until you solve this.......