Besides working as a teacher I freelance as a percussionist in both professional and semi-professional orchestras aswell in smaller ensembles and projects which sometimes uses a conducter. As Rumby put it a conducter does most of his heavy work during the rehearsals, when the actual performances takes place the orchestra knows pretty much what he will do during the piece. Regardless of that he still is very much an important part of the orchestra. In general I would say that in amateur orchestras he's more important as an actual conducter but in professional orchestras his role is to get the most out of the musicians, to push them and motivate them to perform better. Basically turn gold to a diamond in lack of a better word.
With that being said I have multiple times been put on edge when a conducter suddenly feels the urge to do something diffrent by either doing a faster tempo because he wants more energy OR holding a fermata exceptionally long which could totally stump the orchestra if someones not paying attention. That happens from time to time, usually less in professional orchestras but it can be equally funny either way.
The reason why he does something like that and ignores what he did during the rehearsals could be for alot of reasons. A well renowned conducter could do it because he's in the music and moment and wanna create an experience very much like an artist painting a picture from whats feels right at the moment. Therefor he could do drastic changes that could seem rushed if your not on alert but that could also result in certain tension and concentration from the musicians which often leads to magic moments in the music.
Valery Gergiev which is a famous russian conductor, he sometimes uses a conducter that rehearse the orchestra for him so he can arrive directly at the concert. The orchestra is then ready and rehearsed for him to simply create magic in the moment. That's one way of creating tension!
I should say that's not the only reason though, he also do that so he can jump from orchestra to orchestra instead of being stuck for a week at a location. Sounds crazy but he's also not your regular famous conducter. I mean you wouldn't wanna mess with this dude right:
When I was in music school we had the opportunity to be conducted by Okko Kamu which is a famous finnish conducter. We rehearsed monday to thursday and had the concert on thursday night. He was during the whole week very precised on how he was to conducted certain sections in Rimsky-Korsakovs -
Scheherazade and pretty harsh too which lead to everyone being on alert. During the concert he just totally fucking ignored everything we rehearsed and went insane with his "now let's create some magic" so there was some tension to say the least but my god the orchestra sounded good. He forced every musician in the orchestra to be in the moment and to do their absolute best. A very cool yet nerve wrecking experience.
Another reason a conducter diverts from what he did during the rehearsals could also be because he simply forgets what he did on certain sections. In a professional orchestra that usually results in nothing because most are alert and experienced but in a amateur orchestra that could lead to total havoc.
I have countless of times been fooled by the conducters because he promised to cue in my part....
You see as a percussionist the one thing your exceptionally good at in an orchestra is counting bars when your
not playing. Fuck the actual craft, it's the bar counting that's important, slightly exaggerating that part but you get my point.
We for the most part have insane amount of pauses but we still have to know were in the music we are so we can play our parts at the right moment, which sounds a bit strange why you shouldn't do that but trust me you get lost and shit happens. Counting bars is simply a survival thing as a percussionist. If it's a ”classic” piece that you know very well OR music with lots of melodies to follow it's easier to know were you are if you get lost in the counting process because if you've done your ”homework” you have listened alot on the music before and know the sections before your ”part” so to speak. It's much tougher though if the music your playing is a contemporary piece that's freshly baked from the composer and dosen't have reference music. Then your stuck at counting bars but if you then have a nice counducter that actual knowledge the percussion section (which often is not the case but that's another discussion) he knows the important part in your section and helps to “cue” you in. If not then you can always just ask him to cue you in which they for the most have no problem with.
A quick example of another thing that is usually diffrent from classical pieces and modern pieces is that they notate support melodies in the percussion score more often in classical pieces like this for example:
2nd movement in Scheherazade:
That trombone part before is something that's very nice to have highlighted so even if your counting failed you still have something to go after. Btw Piatti=Cymbals
In modern music they're not often so generous to notate support melodies in the percussion score and you could end up with something like this:
That leads to why I said “fooled by the conducter” because he can totally forget your cue sometimes OR he simply ignores your presence entirely which is not a fun experience especially not when you have your cymbals in the ready position, ready to nail that part like a fucking hero man and everyone knows an epic cymbal crash is about to happen, even the audience can see what's about to happen but it dosen't....because the counducter forgets your cue and you stand there like a fool. You know deep down inside that you should've hit the cymbals 3 bars ago, you feel it in your gut. So you quietly sits down and act like nothing happend until you look at your colleagues that's on the floor laughing. Again slightly exaggerated that part, they're pretty reserved during the concert. In the dressing room after not so much.
There's also the reversed version which happens sometimes were you actually nails the cymbal part like a true champ. "Meh he just forgot the cue" is what your thinking in your little bubble and you reacted and saved the situation, you feel good about it, I mean really good until you get a strange face from the conducter and you realise you was 20 bars before the actual part...the reaction from your colleagues is pretty much the same as the first version although slightly more brutal....
The last version is why you hesitate to play the first version without a cue....
but as they say, shit happens!
Sry for ranting!