Gives me a headache - I had a horn band - I was writing the charts for 4 ( Bb, Bb, C and Eb) horns. The guitar player was great but played by ear. He knew the Nashville number system, and could find notes on the fretboard. He like to work little clips of the horn parts into his parts, so I would have to read off the Bb chart and tell it to him in concert. Then the horn players and piano guy would get upset because they couldn't keep up when I was switching between Bb and concert. Piano player could read verbatim and chord charts, but couldn't comprehend why horns played in different keys. Horn players weren't the best. So I was the translator. I could relate to the horn players in their keys and chords, but I had to tell the piano and guitar player to not listen to me, because they would get all confused when I was telling the bari player (even thought the chords were on his chart), what the chords were. I finally got the guitar player to talking with me via the number system, the horn players to understand that I talked to them in their keys, and the piano player ignore everything and just play what was on his chart. The singer, however, she knew music very well, and could rattle off the names of the notes she was singing, and help de- confuse the members when I was talking in horn keys, but they were in concert. Oh, yeah, since we were in flat keys a lot, the guitar player used a de- tuned guitar on some songs. Eventually, the band learned to not listen to me unless I was speaking to them. But the guitar player picked up all of this transposition stuff. Good on him..
The moral is, unless you have a band of well trained players, you will have to establish a common musical language. Sometimes you get lucky, sometimes not so much and working n songs becomes a nightmare.