Via The Morning News, John Adams gives tips on getting through the first rehearsal:
Be flexible and take every opportunity to talk to the players. Sometimes you can make an on-the-spot change that will make an instrumentalist’s day. Other times, although you realize that what you’ve written is in fact awkward and unreasonable, the player will be affronted if you offer to simplify or revise a phrase or a passage. They assume that if something isn’t working it’s their fault. Composers are geniuses, right? For them it’s their burden to somehow make it work, and they do not realize that it’s the composer who needs to get it right.
Many actors I know aren’t quite so accommodating toward playwrights.
I also must quote this bit, relating to the many, many details of the composition process:
[Is] it mezzo forte or mezzo piano? (Or maybe pianissimo, because they’ll play it loud in any event?) Why aren’t there more gradients available? (Stockhausen tried: zero to ten.) Is pianissimo in the brass still going to cover the clarinets? And you always forget about mutes. It says “mutes on,” but you’ve declined to say when to remove them. Is it true that Schoenberg thought “mezzo” forte and “mezzo” piano were for sissies who couldn’t make up their minds? Maybe he was right.