An interesting bunch of replies, so I thought I might add my own "two-penn'orth". Many years ago I did an extended spell as a "depping" organist - deputising for organists who were sick, on holiday, etc. This involved travelling to strange clubs, confronting (often) strange organs, and accompanying "cabaret" with a strange drummer. For anyone who has not actually done this, I can tell you it is a "school of hard knocks", where you quickly learn new musical skills or perish. Often the "artistes" would ask for the music to be played in a different key to the dots - and this long before the advent of keyboards with transpose buttons. Usually we (me and the drummer) had about 5 minutes to understand what the artistes required and then we "were on".
About the same period, I had a weekly live half-hour show playing organ on local radio. This, again, was truly hair-raising at first. The idea was that I played requests as they came in during the program from telephone calls. There was no question of sorting out the dots - too slow - so I had to either play the requests or admit (on the air) that I didn't know them. Well, you can't do the latter too often and keep your spot. I can truly say that those experiences were the ones where I really learned my craft.
Mike O'R