@mjmooney & @lapal_fan You are both absolutely right with what you say. Maybe I can throw a tiny bit more nuance on things that I've learnt from my time "under the baton."
As I tell my students, the first rule of being a professional musician is Never look at the conductor
Although thats a joke, there is an element of truth to it. In terms of keeping time, or as I prefer to think of it, maintaining the pulse of the music, indulge me: When a solo instrumentalist performs they are solely dependent on their internal pulse. The audience has their own pulse too and it is up to the artist to make sure that they communicate a strong pulse to the listener. There is nothing more awful than listening to a soloist who has a poor sense of pulse. The listener must be lulled and wooed by the pulse that the soloist is communicating.
A smaller ensemble, say a string quartet, relies on a "group pulse" which is generated by listening intently to ones colleagues and being aware of when you should be directing the time (i.e. a set of running semi-quavers would mean that the rest of the group should usually be listening to you at that point.) There is also something of a "sixth sense" that develops in this kind of ensemble playing. The pulse and time is very democratic and the ensemble will fail if one person takes it upon themselves to dictate the tempo constantly to the rest of the group.
The largest non-conducted chamber ensemble I've come across (apart from jazz ensemble with rhythm section) would be 8 players. Beyond that you are looking at having a conductor. There are several notable "conductor-less" string ensembles of around 17 string players. These entail are large amount of leading (almost like conducting) from the concert master (lead violin.) My wife incidentally co-founded a very successful conductor-less string ensemble in Boston around 14 years ago - A Far Cry.
But beyond 8 chamber players, or a heavily led string ensemble, this is when conductors ruffle their hair and ascend the podium.
So what does a conductor do? I would say the main bulk of their work happens during rehearsal. Sure they decide general tempi in the show, but depending on their level of competence, in rehearsals they will talk about balance issues, phrasing, bowings (how the strings use their bows) bow speed, quality of sound and articulation plus a million other minutiae that make up a symphony. That then becomes their "interpretation" of the composers intentions and it is becoming, sadly, something of a dying art-form.
Here is John Barbirolli working with the Halle Orchestra on a Bruckner Symphony in rehearsal form the early 60's. He knew what he wanted even if his stick technique definitely falls into the "Never look at the conductor" realm.
I was fortunate enough to know his old 1st Trumpet player in the Halle when I was a kid and he did a great impression of Barbirolli. Sir John also kept a bottle of whiskey at the side of the stage and would always take a swig before he walked out. Thing was, so would half of the orchestra. When he realized, he used to mark the bottle with a marker pen
Alas these days, conductors don't spend much of time with their orchestras. Through the 50's & 60's conductors would spend the majority of the season in their home town, cultivating a sound and a style that was unique. You had Barbirolli in Manchester, George Szell in Cleveland, Ormandy in Philadelphia, Reiner in Chicago and of course Bernstein in New York.
With the advent of easier jet travel through the 80's conductors started to realize that they could have not just 1, but 2 or 3 orchestras (and pay-checks) so the connection started to get lost - as did the quality. Conductors spent less and less time with their "home-town bands" and most Directors now only spend 10-14 weeks per year with their ensembles. In fact I believe when Sir Simon Rattle left the City of Birmingham Symphony Orchestra, he was the longest serving Music Director of any European orchestra. And probably the last one that really forged a unique sound and identity. We were lucky to have him!
Back to what they actually do - conductors of course set the tempo at the beginning of a piece in performances and also set the interest level of the musicians on stage! If he/she has brought something to the table during the week of rehearsals you'll see an excited group ready to perform. If someone showed up who was a jerk, or out of their depth/wet behind the ears, the group will be phoning it in.
Orchestras need conductors for what we call the "corners" of music. When you get to a large rallentando, or a large climax, that is when they need to have good stick technique and clearly show that section of the music. That is what they are there for in performance - the rest is directing traffic and the really pretentious ones move around a lot and flick their hair etc.
One of the best I was privileged to work under was the late great Sir. Colin Davies. Going back to the original post, I also played under John Williams at Tanglewood with the Boston Pops a few years ago. -That was cool as ****. Just to back up some of the stuff I've been saying, we didn't even rehearse the Star Wars suite! - we'd all played many times before and he'd conducted it a million times. He is probably sick of it the truth be told even though it's made him a multi millionaire!
I'm glad you like the show @lapal_fan Check out some Prokofiev or Shostakovich (e.g 5th Symphony) - John Williams ripped off both of those guys! Also he stole a lot of Mahler - thats was the composer that showed me how powerful, profound and moving orchestral music could be. If I hadn't heard the opening to Mahler 5 when I was 13 my life would be very different. Just check out the 1st couple of minutes of this.
This is my area and my life so I apologize for the thesis. But you asked - That'll learn ya!
