Hi Rok - answers to your questions will vary widely according to the conductor involved. They control what happens in rehearsals. Ideally, a dress rehearsal of an opera is indeed a complete run-through, though depending on how close to the performance it is, the lead singers may be "marking," in other words, not singing full out. This depends on the rehearsal and performance schedules, and the individual singers.
Ideally, a dress rehearsal of a symphonic performance is also a complete run-through, though if it is on the same day as the first performance, as it so often is, the brass section in particular will not be playing full out. I personally hate dress rehearsals on performance days, but that is almost always when they are. This is actually normally due to the soloist's schedule. Usually they don't come in until the day before, for the second to last rehearsal, then there is the dress the morning of the first show.
As for the orchestra not meeting expectations, again this depends on the conductor and the ensemble. No one is perfect - many mistakes are made in almost every performance, though the vast majority of them will go completely unnoticed by almost the entire audience. One of my teachers said once - batting .300 is great for baseball, but doesn't cut it in music, which is hilarious. But absolutely no one is note perfect all the time. In my opinion, far too much emphasis today is placed on not making any mistakes, rather than really making music. This is due to a couple of different factors - the greatly increasing technical proficiency of young musicians coming out of school - already far, far higher than when I came out of school in the late 80's - and also the availability of so many recordings now where mistakes have been digitally edited out. This has increased pressure on live performers greatly, again leading to players just trying not to make any mistakes rather than really getting into the music.
Often the very first rehearsal of a symphony will be a complete run-through, too, by the way. The players usually like this very much, as they see the conductor's tempi, etc. right off the bat, and get a feel for how the piece is going to go. Not all conductors will do this, however, again every one is different. Some have great time management but are not musically very good, others have horrible time management in rehearsals, which usually leads to sections of the music feeling very uncomfortable in the first performance. Very rarely are great musicianship and great time management present in the same conductor. As far as a conductor's expectations, it is their job to show this as much as possible with their conducting (which is all they can do in a performance), though of course they stop and talk a lot in rehearsals, too. Again, the quality and effectiveness of what they have to offer varies quite a bit. Some conductors have much better ears and are better at fixing certain kinds of issues than others, etc. Bad conductors will also often try to micro-manage everything, instead of letting us do what we do. Young conductors often fall into this trap, even though most of the orchestra knows the pieces way better than they do. Other times many conductors think they have to re-invent the wheel in an often played work, and will do stupid things that don't make much sense. Or they might make a horrible mistake themselves in a performance. In almost every case, the orchestra will save their butts, unless they really don't like the guy. There is a very famous story about the Boston Symphony letting a performance of Ein Heldenleben come to a screeching halt because they really hated the conductor, who was sky high on coke on the night, as he often was. The conductor in question is a VERY big name, and is indeed a good musician, though. Bottom line is - don't assume the conductor always knows what they are doing. Another kind, by the way, is one who may know exactly how they want something to sound - perhaps they are a great rehearsal pianist, for instance, but they can't actually conduct to save their lives. They have no physical ability to communicate to a large ensemble what they actually want to happen musically (or even time-wise, in the worst cases). There are many ballet and opera conductors like this out there.
I believe some time ago in this same thread we discussed job security issues - all union orchestras have a peer-review board type of system in place, so conductors can't just fire someone for no real reason, much like lawyers and doctors have. Normally, though, the process doesn't actually get that far - something is usually worked out to where the musician "retires" early. There is also a tenure process in all orchestras, usually one, sometimes two years in length. If at the end of that period the recently hired musician is deemed not up to snuff, they are not tenured in the first place.
Hope this answers your questions reasonably.
