To sub or not to sub, that is the question

Subbing is one of those things that can only do one of two things: open doors, or close doors.

The nature of subbing could easily trick us into thinking that whatever we do won't weigh as much for our reputation as our regular gigs, but it turns out that it's often right there that people will judge our value.

Say yes to one subbing gig and you'll be called again and again for more, say yes to another and your hard-earned reputation will start being doubted.

WHEN NOT TO SUB

1. Conducting: if you're asked to be the sub conductor past the first two or three rehearsals, and you say yes, here is what will happen: you won't know the music as well as the musicians because most likely you won't have had the preparation time you would have needed to be ready (if you ever got the score in advance at all).
   
   Even if you have already done the piece before, you will never take the exact same tempo as the regular conductor. That shouldn't be a big deal, but it always ends up being a source of frustration for the players. Any ritard, accelerando, and tempo change would also not be like the musicians are used to, along with pretty much anything stylistic. You just can't win.
   
   
2. Concert: besides for a few exceptions, do not sub for a concert. Never even consider doing it unless you have a score in hand to assess the situation. Do not sub for a concert if you have major solo lines, say high trumpet line in a duet with the singer in a musical.
   
   Do not sub for a concert if you would be responsible of setting the tempo at any given time of the concert: so don't do it as an accompanist, don't do it as first violin of a chamber group, don't do it as a conductor.
   
   
3. Sight-reading: I break that rule all the time, and every so often I'm given a piece to sight read that is clearly not sight-readable. Even if I manage quite alright given the circumstance, there is always someone there to tell me that I missed that one dynamic spot, or that in that section it's more of a rhapsody feeling, or some kind of something.
   
   Sadly, no one ever considers sight-reading in a different light than performance-ready, and since the point of sight-reading is that you don't see the score ahead of time, you just can't know in advance if it's gonna do you good or harm you.
   
   
4. Dress rehearsal: for all the above reasons mentioned, subbing for a dress rehearsal won't work. It's again an issue of tempo, style, and on top of it, everyone involved will become really stressed for the concert because their last rehearsal will have been different from what they're used to do. 

WHEN TO SUB

1. Conducting: if you have been playing with the actual group that needs a sub, and you have been at all the rehearsals and know all the tempi and everything the regular conductor wants, then you stepping in will work fine.
   
   And because the rest of the musicians will already know you, they won't test you like they would a new person. You will help the entire group and everyone will be grateful.
   
   
2. Concert: you can sub for a concert if you're the only one playing, so basically, if you are a solo pianist!
   
   You also can if you are playing the same line as other players around you, say you're another cellist in a symphonic orchestra. You can sub for a concert if you know that no one will be in the audience (student recitals anyone?).
   
   
3. Sight-reading: it's ok if you're very experience in a particular rep and you've been in similar circumstances before. For example, I've played for a ton of music theatre classes, so I would sub for one in a heart-beat.
   
   Same with college auditions for singers. Some of the songs to sight-read at auditions can be tricky at times, but because of the volume of music played at each audition, the ratio of un-sight-readable song to well-done song might be around maybe 5%, so you will be forgiven and still considered an excellent musician.
   
   
4. Church service: those are usually fine to sub for, as long as you make sure there's not last minute sight-reading of say, a Bach cantata solo or four-part vocal counterpoint.
   
   
5. Choirs: fine to sub for unless they fall in the categories of conducting, dress rehearsal, or concert.
   
   
6. Cocktail parties: yes, always. Except for that one time someone gave me a Chopin waltz to sight-read on the spot, I never had anything weird happen at cocktail parties! And they're a ton of fun. 

Photo from http://www.ehow.com/how_5596941_become-substitute-teacher-baltimore-city.html