Truth be told, theatrical music (opera, Broadway and West End shows, and the kind of stuff that my group plays) is not for the faint of heart. It's a whole different world in which the musician is only a part, not the whole.
Whereas the typical jazz gig is all about the musician (i.e., the players set the tempo, the style and what have you), all of theatrical music is support for something else. Suddenly, the musicians are no longer in control, and without some care this is a recipe for musical disaster.
In our case, it's only the vocalists, and both sides adjust and accommodate each other to make it work. It's not impossible to make work, even if it is a bit of a problem getting it set up through rehearsal.
In the others, it a lot more complicated. Part of the difficultly arise from the methods of rehearsal, part of them from negligence/ignorance on the part of the vocal performers/actors, and part of it from poor communication between all parties involved. But, it is a lot more complicated than setting up a jazz trio and calling a tune.
Rehearsal techniques fail since they always (well, almost always) use a rehearsal pianist as the source of music. This worthy is always too accommodating to the vocalists, and in effect ends up conditioning them to not worry about tempo, note duration and the like.
The performers, in turn, have their own problems. Although we in the music world tend to poo-poo their objections, you have to realize that they are doing three or four things at once, and trying to make it all fit musically. To perform before a house full of spectators is more than most of us can do; to do it while dancing and "acting" (whereby, every night, you have to reproduce your actions, timing and movements in the same rigid fashion, all while making it look effortless) is exponentially more difficult.
Finally, you have the battle of personalities. The musical director of a show is more than just a musician who has a few contacts and credits under his belt. In addition to working up the pit orchestra (usually through a series of three or four rehearsals), he also has to coordinate with the other "directors" (only a few of whom are credited as such) to ensure that the music works with everything else.
Like everyone else, I hate technical rehearsals (where the problems are worked out), since it involves sitting around for two hours and playing for only one. However, this is where the stage direction and the musical director earn their money, as getting it all to gel is what makes the show worth the time (and money).
Then, there's the wonderful world of live performance. I'm not talking about missing a change here and there, but rather the fact that all of the music is hostage to what's going on up on the stage.
Those of us who play in pit orchestras all have a favorite musical horror story to tell about how this interplay between the music and the show has gone wrong. My favorite is a production of Hello Dolly! back in the 1970's that I did up in Saint Louis. The "eleven o'clock" number (where Dolly is brought in down that big stairway) is either followed or preceded by a dance number called the "Waiter's Gallop", which in turn segues into a dance competition up on the stage involving a lot of comedic business.
The "Gallop" is fun to play, with a lot of quick stuff on soprano clarinet that requires split second timing. But, after all of that speeding around, the Reed IV player then picks up his bass clarinet and plays a simple vamp figure, accompanied only by the string bass desk.
It's not hard to play, only a single bar in 2/4 time with an upward arpeggio followed by a bump on the second beat. It was inserted to allow for previous participants in the "Gallop" to get in position, catch their breath (the "Gallop" is very strenuous), and change out props and costumes for the dance competition.
Usually, I have ended up playing the vamp maybe five or ten times. On the night in question, however, there had been a serious wardrobe malfunction backstage, this involving a cast member that was essential to the plot at that point. No costume change equalled a shot scene in the show, making it all a catastrophic failure.
So, there I sat, going "barrump-bump" on the bass clarinet while needle and thread were employed to make it all right backstage. (No safety pins were available, apparently.) I lost track of how long we two filled in musically (while the actors on stage did a workman like job filling in with ad libs and cute stage business), but it finally ended before the two of us went completely insane from the repetition.
The director of the play bought us both a drink that evening (or, rather, bought the bass player two drinks, since I don't imbibe.) And, I can play that one measure from memory even today (twenty or so years later), so it wasn't all a waste.
Aside from little problems like this, there are the times that the vocalists decide to drop whole choruses from what they're slated to sing, of very good actors who can't hold a pitch to save their souls, and when someone swans off of the stage and ends up in the percussion section. Or, when a bathtub and water bit on stage ends up dousing the string desks when someone climbs in a bit fast. Or, when the electrical power in the pit goes out and you are stuck reading manuscript charts from the theatrical lighting.
All in all, it's a challenging musical world in which to be operating. And, to add insult to injury, you are never the center of attention as the musician, the glory instead going to the actor or vocalist. Still, as an integral part of the whole thing, it's rewarding enough. And, it's one version of music where the clients are still willing to pay real money for your services.