ABitRusty said
in reality.. when will a orchestral or ensemble ...quartet ..muscian have to on the fly read and play a piece of music for performance thats not been practiced?   Other than an audition.

I can see that happening in like an recording session for an album or movie or something like that.   a person takes a gig, grabs his cello and goes down to Abbey Road and is given his part which they run through, record then he goes home.  

gigging muscians ..probably the truest form of sight readers?  Im speculating.

  

Film and TV studio orchestras sight-read in recording sessions on a regular basis. They rarely get more than one rehearsal, and the rehearsal is usually only long enough to hit a few important spots, so large portions of the music are being sight-read in the recording.

Although an orchestra sight-reading an entire piece in concert is rare, performing on short rehearsal time is very common. Sight-reading is an essential skill because a large amount of music needs to be learned in a short time.

Things like wedding gigs (or other events) are rarely rehearsed at all; a wedding quartet has typically rehearsed the most commonly requested music, but often if something unfamiliar is requested they either quickly find an arrangement or have one of the musicians make an arrangement, and sight-read it at the gig. Also, if they need a substitute musician, often the sub is sight-reading everything.

As an amateur, I do a lot of sight-reading in first rehearsals. My orchestra typically distributes music about a week in advance of the first rehearsal, but realistically there isn't time to practice everything. Before the first rehearsal, I look over the music and figure out what needs practice in advance, so typically I've practiced the tricky-looking passages but nothing else. I do listen to recordings if possible, but most of what I get out of it is familiarity with what other parts sound like and when I should start playing. It's often of limited use for learning my own notes because the viola part may be in the background. (Also, listening to recordings is not always possible because some of the music we play is contemporary music that has never been recorded.)

Many community orchestras distribute parts in the first rehearsal, which means everyone is sight-reading everything in that rehearsal.

Sight-reading is also something I do for fun quite often. I occasionally meet up with other musicians for casual chamber music reading sessions, and sometimes I look through IMSLP and sight-read pieces to see if I'd be interested in learning them more thoroughly.

If you follow my blog here, you'll notice that this week I was just at a week-long chamber music workshop, where I was starting a new piece with a new group every morning and performing in the evening the same day. For this kind of workshop, sight-reading is probably more important than technique. Some of the pieces I played this week were well-known pieces that everyone in the group had heard before, but during the week I was assigned two pieces to perform that I'd never heard of. In the evenings, I did some of what the workshop calls "freelancing", which is just casual reading with self-formed groups -- and in those sessions I played three pieces I'd never heard before. Even at an amateur workshop, in the four times I've attended, I've played in several groups that could stay together and play almost all the notes in tempo in their first reading of a piece, and go straight into working on interpretation.