I think you mean “playing loudly when the conductor is indicating softly.” A conductor would never indicate that you “play sharp.” It would be telling you to play the wrong notes.
Thank you all for the explanation! The comment about the director bringing together people for the movie really helped.
You also would have received full credit for, “they drive the train.”
Considered it, believe me.
But the electrical answer reminds me of one of the best “bad jokes” I was ever told, so I went with it.
What do conductors do?
Yell, throw shit and storm off, best as I can tell.
The guys get shirts! That’s just the way it is!
Eh, playing sharp would sound nasty, too. 
The thing is that when you have a 100 piece orchestra and you are sitting in the middle of it, you can’t hear everything or, more importantly, how everything is blending. Sure that guy right behind you sounds loud but he may still be being burried by the string section which you can barely hear.
To add to other replies to this: every performance is different. By which, I mean every single time a piece is played, it’s different from before and different from what will happen next time. This is one of the many things that makes a live performance so different from a recording.
It was a short-lived project with one Moscow orchestra in the early years of Russian communism, where they tried to apply the collective principle to an orchestra. Any and every member of the orchestra could make a comment (the second clarinet could suggest a change of the cellos’ bowing, for example), and a vote would be taken. Three guesses why this was short-lived!
The only experience I’ve had of truly great conductors is watching them. If you get the chance, buy cheap seats in choir stalls - you’re behind the orchestra, so it sounds awful, but you get to see just how much the conductor does, and how much silent communication he has with every part of the ensemble.
Cheesesteak had a very good answer, that no one else has touched on yet.
The same piece of music in the hands of different conductors can sound dramatically different, just as the same Chopin Nocturne will sound different as played by different pianists. Mood and feeling are all expressed musically, and the conductor not only decides what that mood and feeling will be and how it is expressed, but knows how to direct the players to get that exact mood.
To add to Eonwe’s… uh, addition, the reason a conductor decides this is because sheet music for any particular piece conveys a limited amount of dynamic information. The notation conveys meanings for soft, loud, legato vs. staccato (notes smoothly flowing vs. notes that are distinct and separated), and so on, but there’s no objective standard for how soft pianissimo is or how smooth legato is.
It’s just like the difference between words in a script, and how an actor might read them. The script only conveys so much data about how to read the line.
Excuse a slight hijack, but it does say something about orchestra conductors.
When back east,we used to go to Tanglewood in Mass. for performances, and also to stop by the great Conductors School they had there. One time we even enjoyed seeing Bernstein teaching there, and he was wonderful.
Anyhoo, to the point. There was a story about one time the school had a special event where each student conductor was given a chance to lead the orchestra in short piece. It was in the evening on an outdoor stage. The stage was decorated not only with footlights, but also with strings of colored lights above the stage platform.
One young conductor was extremely lively, jumping and leaping in the air through his performance. At one point, he leaped in the air and his baton came in contact with a part of the lighting wire that was scraped bare, and a huge flash of light resulted as the current went down the baton, down his arm and body to the ground.
Fortunately, he was not hurt at all, as he was such a bad conductor.
Exactly. Scores will often say “accel.” (speed up) or “rit.” (slow down), but won’t say exactly how MUCH to slow down. In a group of 10 people or so, they can all “feel it” together, but anywhere above that, it won’t happen.
Perhaps the conductor’s most important job, though, is dealing with vamps in musicals. A vamp is some music, usually a measure or two, which is repeated over and over during someone’s dialogue. When the actor gets to a certain place in the dialogue, the conductor has to tell the orchestra to come out of the vamp, otherwise the different musicians will be at a different spot in the music. Then, they have to indicate to the actor that the vamp is over and it’s time for them to start singing. I kind of enjoy watching the conductor, and sometimes you’ll see him use a particular hand movement for singers that he doesn’t use for any other musician.
It’s the safest bet in town…as long as I’m still breathing. 
Do conductors ever change the way they will conduct a particular piece, just on a whim? Do any conductors, for example, decide to make the strings extra loud or soft, just for the heck of it? I guess what I’m asking is do they ever “improvise,” and surprise the musicians?
Not during a performance, but many times during a rehearsal a conductor may mix things up a bit, usually to make sure we are paying attention to him and not just getting lost in the printed page.
The conductor is the one person who has the overall vision of how the piece should sound, and he or she teaches everyone how to attain that sound. And then keeps working at it and tweaking it until everyone knows their part and “feels” the same things. He has to know every part for every voice or instrument, and know which ones are the most important at every moment. The individual musician only has to worry about their single part. The conductor’s real work comes during rehearsals.
And a bad conductor can ruin the work of a well-trained ensemble. Many times, when doing collaborative pieces, or when a guest conductor will be appearing, we train for weeks with one conductor, who sometimes has received specific instructions on how to approach the work. We learn the piece, we have it down cold, and we sound fantastic. In walks a new director, and we perform as if we’ve never seen the score before. We can’t follow the conductor because we aren’t familiar with his way of conducting. He may use a different motion for cut-offs that is indistinct. He may be extra flourishey, which leads to confusion. Or she may be timid and not cue entrances clearly. The conductor can make or break the performance, no matter how brilliant the musicians are.
Hmm my recollection from high school band. He’s the guy who goes:
“Flutes. FLUTES where are my darn FLUTES!!! STOP! STOP! Sweet Lord are you all asleep? I feel like I’m in junior high. You sound like a pod of tone deaf whales!!! And second clarinets? I know none of you have been practicing. Do you realize the concert is in two weeks??? And the only one who knows the music is first saxophone. Do any of you care??? I’m thinking of cancelling the concert. How would you all feel if I just cancelled? Do you really want to get in front of the whole school and embarrass yourselves this way? Timpany do you have something to share with the whole band?..” etc.
Uglybeech, that sounds like me, in, errrrm, about three hours time 
Ever after many years of playing in ensembles, I don’t think I really understood how important a conductor was until a particularly good one at music camp gave us a little Intro to Conducting course. The conductors don’t just wave their hands about, there really is a method to their hand motions.
This conductor had us conduct along with him during our rests, or when he had other sections play. I have a particularly bad sense of time (surprise, surprise, I’m lousy at sightreading) and this helped me immensely.
Hold your hand up in front of you. Now lower it about a foot (count “one”). Move it off to the left (count “two”), and then to the right (count “three”). Now move it back up to the starting position (count “four”). You should be tracing a t in the air, in 4/4 time, a very common time signature.
Try it some time, and you’ll feel the beat and the shifts in the tempo immediately.
As a drummer, getting a cue from the conductor can be vital. I have played concerts where I literally had 10 notes in the entire concert, with rests lasting 200+ measures. It is also vital to lock in with the conductor because the distance back-to-front can cause just enough of a delay to throw everything off. Kinda like phasing back-to-front across a marching band on a football field. This goes double when you’ve got the melody or the beat. And in modern music, there can be sections of free time–for a soloist, say. The conductor’s got to get everyone out of that free time together. A conductor is also vital when a full group is the accompanient–imagine a full orchestra trying to play accompanient for Rhapsody in Blue without a conductor to match the tempo of the pianist or indicate enterances.